What is PHP?

PHP is a server side scripting language. that is used to develop Static websites or Dynamic websites or Web applications. PHP stands for Hypertext Pre-processor, that earlier stood for Personal Home Pages.

PHP scripts can only be interpreted on a server that has PHP installed.

The client computers accessing the PHP scripts require a web browser only.

What is a Scripting Language?

A script is a set of programming instructions that is interpreted at runtime.

A scripting language is a language that interprets scripts at runtime. Scripts are usually embedded into other software environments.

The purpose of the scripts is usually to enhance the performance or perform routine tasks for an application.

Server side scripts are interpreted on the server while client side scripts are interpreted by the client application.

PHP is a server side script that is interpreted on the server while JavaScript is an example of a client side script that is interpreted by the client browser. Both PHP and JavaScript can be embedded into HTML pages.

What does PHP stand for?

PHP means – Personal Home Page, but it now stands for the recursive backronym PHP: Hypertext Preprocessor.

PHP code may be embedded into HTML code, or it can be used in combination with various web template systems, web content management system and web frameworks.

Why use PHP?

You have obviously head of a number of programming languages out there; you may be wondering why we would want to use PHP as our poison for the web programming. Below are some of the compelling reasons.

  • PHP is open source and free.
  • Short learning curve compared to other languages such as JSP, ASP etc.
  • Large community document
  • Most web hosting servers support PHP by default unlike other languages such as ASP that need IIS. This makes PHP a cost effective choice.
  • PHP is regular updated to keep abreast with the latest technology trends.
  • Other benefit that you get with PHP is that it’s a server side scripting language; this means you only need to install it on the server and client computers requesting for resources from the server do not need to have PHP installed; only a web browser would be enough.
  • PHP has in built support for working hand in hand with MySQL; this doesn’t mean you can’t use PHP with other database management systems. You can still use PHP with
    • Postgres
    • Oracle
    • MS SQL Server
    • ODBC etc.

PHP Data Types

A Data type is the classification of data into a category according to its attributes;

  • Alphanumeric characters are classified as strings
  • Whole numbers are classified integers
  • Numbers with decimal points are classified as floating points.
  • True or false values are classified as Boolean.

PHP Variable

A variable is a name given to a memory location that stores data at runtime.

The scope of a variable determines its visibility.

A Php global variable is accessible to all the scripts in an application.

A local variable is only accessible to the script that it was defined in.

Think of a variable as a glass containing water. You can add water into the glass, drink all of it, refill it again etc.

The same applies for variables. Variables are used to store data and provide stored data when needed. Just like in other programming languages, PHP supports variables too. Let’s now look at the rules followed when creating variables in PHP.

What is a PHP Array?

A PHP array is a variable that stores more than one piece of related data in a single variable.

Think of an array as a box of chocolates with slots inside.

The box represents the array itself while the spaces containing chocolates represent the values stored in the arrays.

Numeric Arrays

Numeric arrays use number as access keys.

An access key is a reference to a memory slot in an array variable.

The access key is used whenever we want to read or assign a new value an array element.

PHP For Loop

The above code outputs “21 is greater than 7” For loops For… loops execute the block of code a specifiednumber of times. There are basically two types of for loops;

  • for
  • for… each.

While Loop

PHP While loop

They are used to execute a block of code a repeatedly until the set condition gets satisfied

When to use while loops

  • While loops are used to execute a block of code until a certain condition becomes true.
  • You can use a while loop to read records returned from a database query.

Types of while loops

  • Do… while – executes the block of code at least once before evaluating the condition
  • While… – checks the condition first. If it evaluates to true, the block of code is executed as long as the condition is true. If it evaluates to false, the execution of the while loop is terminated.

What is a string?

A string is a collection of characters. String is one of the data types supported by PHP.

The string variables can contain alphanumeric characters. Strings are created when;

  • You declare variable and assign string characters to it
  • You can directly use them with echo statement.
  • String are language construct, it helps capture words.
  • Learning how strings work in PHP and how to manipulate them will make you a very effective and productive developer.

What is a Function?

A function is a reusable piece or block of code that performs a specific action.

Functions can either return values when called or can simply perform an operation without returning any value.

PHP has over 700 functions built in that perform different tasks.

Why use Functions?

  • Better code organization – functions allow us to group blocks of related code that perform a specific task together.
  • Reusability – once defined, a function can be called by a number of scripts in our PHP files. This saves us time of reinventing the wheel when we want to perform some routine tasks such as connecting to the database
  • Easy maintenance- updates to the system only need to be made in one place.

String Functions

These are functions that manipulate string data, refer to the article on strings for implementation examples of string functions

Numeric Functions

Numeric functions are function that return numeric results.

Numeric php function can be used to format numbers, return constants, perform mathematical computations etc.

What is Form?

When you login into a website or into your mail box, you are interacting with a form.

Forms are used to get input from the user and submit it to the web server for processing.

When and why we are using forms?

  • Forms come in handy when developing flexible and dynamic applications that accept user input.
  • Forms can be used to edit already existing data from the database

Create a form

We will use HTML tags to create a form. Below is the minimal list of things you need to create a form.

  • Opening and closing form tags <form>…</form>
  • Form submission type POST or GET
  • Submission URL that will process the submitted data
  • Input fields such as input boxes, text areas, buttons,checkboxes etc.

Submitting the form data to the server

The action attribute of the form specifies the submission URL that processes the data. The method attribute specifies the submission type.PHP POST method

  • This is the built in PHP super global array variable that is used to get values submitted via HTTP POST method.
  • The array variable can be accessed from any script in the program; it has a global scope.
  • This method is ideal when you do not want to display the form post values in the URL.
  • A good example of using post method is when submitting login details to the server.

PHP GET method

  • This is the built in PHP super global array variable that is used to get values submitted via HTTP GET method.
  • The array variable can be accessed from any script in the program; it has a global scope.
  • This method displays the form values in the URL.
  • It’s ideal for search engine forms as it allows the users to book mark the results.

GET vs POST Methods

Values not visible in the URL Values visible in the URL
Has not limitation of the length of the values since they are submitted via the body of HTTP Has limitation on the length of the values usually 255 characters. This is because the values are displayed in the URL. Note the upper limit of the characters is dependent on the browser.
Has lower performance compared to Php_GET method due to time spent encapsulation the Php_POST values in the HTTP body Has high performance compared to POST method dues to the simple nature of appending the values in the URL.
Supports many different data types such as string, numeric, binary etc. Supports only string data types because the values are displayed in the URL
Results cannot be book marked Results can be book marked due to the visibility of the values in the URL

What is PHP mail?

PHP mail is the built in PHP function that is used to send emails from PHP scripts.

The mail function accepts the following parameters;

  • Email address
  • Subject
  • Message
  • CC or BC email addresses
    • It’s a cost effective way of notifying users on important events.
    • Let users contact you via email by providing a contact us form on the website that emails the provided content.
    • Developers can use it to receive system errors by email
    • You can use it to email your newsletter subscribers.
    • You can use it to send password reset links to users who forget their passwords
    • You can use it to email activation/confirmation links. This is useful when registering users and verifying their email addresses.

What is OOPs?

Object Oriented is an approach to software development that models application around real world objects such as employees, cars, bank accounts, etc. A class defines the properties and methods of a real world object. An object is an occurrence of a class.

The three basic components of object orientation are;

  • Object oriented analysis – functionality of the system
  • Object oriented designing – architecture of the system
  • Object oriented programming – implementation of the application.

Object Oriented Programming Principles

The three major principles of OOP are;

  • Encapsulation – this is concerned with hiding the implementation details and only exposing the methods. The main purpose of encapsulation is to;
    • Reduce software development complexity – by hiding the implementation details and only exposing the operations, using a class becomes easy.
    • Protect the internal state of an object – access to the class variables is via methods such as get and set, this makes the class flexible and easy to maintain.
    • The internal implementation of the class can be changed without worrying about breaking the code that uses the class.
  • Inheritance – this is concerned with the relationship between classes. The relationship takes the form of a parent and child. The child uses the methods defined in the parent class. The main purpose of inheritance is;
    • Re-usability– a number of children, can inherit from the same parent. This is very useful when we have to provide common functionality such as adding, updating and deleting data from the database.
  • Polymorphism – this is concerned with having a single form but many different implementation ways. The main purpose of polymorphism is;
    • Simplify maintaining applications and making them more extendable.

What is PHP Date Function?

PHP date function is an in-built function that simplify working with date data types. The PHP date function is used to format a date or time into a human readable format. It can be used to display the date of article was published. record the last updated a data in a database.

What is Ajax?

AJAX is the acronym for Asynchronous JavaScript & XML.

It is a technology that reduces the interactions between the server and client.

It does this by updating only part of a web page rather than the whole page.

The asynchronous interactions are initiated by JavaScript.

JavaScript is a client side scripting language. It is executed on the client side by the web browsers that support JavaScript.JavaScript code only works in browsers that have JavaScript enabled.

Why use AJAX?

  • It allows developing rich interactive web applications just like desktop applications.
  • Validation can be performed done as the user fills in a form without submitting it. This can be achieved using auto completion. The words that the user types in are submitted to the server for processing. The server responds with keywords that match what the user entered.
  • It can be used to populate a dropdown box depending on the value of another dropdown box
  • Data can be retrieved from the server and only a certain part of a page updated without loading the whole page. This is very useful for web page parts that load things like
    • Tweets
    • Commens
    • Users visiting the site etc.

How to Create an PHP Ajax application

We will create a simple application that allows users to search for popular PHP MVC frameworks.

Our application will have a text box that users will type in the names of the framework.

We will then use mvc AJAX to search for a match then display the framework’s complete name just below the search form.

PHP vs JavaScript: Must Know Differences


Is not fair to compare PHP vs JavaScript, as they both have different purposes for web-site development. PHP is a server-side scripting language while JavaScript is a client-side scripting language. In fact, the most dynamic website is created when we use functions of both these languages together. If PHP is like a paint-brush to paint picture, then JavaScript is a paint-color.

PHP stands for “Hypertext Preprocessor”, is a programming language embedded in HTML that does all sort of things like build custom web content, send and receive cookies, evaluate form data sent from a browser, etc. It is integrated with number of popular databases like Postgre SQL, Oracle, Sybase, SQL, and MySQL. PHP also supports major protocols like IMAP, POP3 and LDAP.

PHP can handle forms, save data to a file, return data to the user, gather data from files, etc.

Example: Let say a website that takes user to view the order status after logging in. By PHP coding, you would send a query to the database that would then output the specific user information based on what information is in the database


While, JavaScript is designed for creating network-centric applications. With JavaScript, web pages will no longer be static HTML and allows the program that interacts with the user, control the browser, and dynamically create the HTML content. The advantage of JavaScript is that it has less server interaction, allowing you to validate user input before sending the page off which means less load on your server and less server traffic. JavaScript allows immediate feedback to the visitors.

Example: When you hover over the menu tab on the web-page, the drop down effect is done through JavaScript.

Top 75 PHP Interview Questions and Answers

1) What is PHP?

PHP is a web language based on scripts that allow developers to dynamically create generated web pages.

2) What do the initials of PHP stand for?

PHP means PHP: Hypertext Preprocessor.

3) Which programming language does PHP resemble?

PHP syntax resembles Perl and C

4) What does PEAR stand for?

PEAR means “PHP Extension and Application Repository”. It extends PHP and provides a higher level of programming for web developers.

5) What is the actually used PHP version?

Version 7.1 or 7.2 is the recommended version of PHP.

6) How do you execute a PHP script from the command line?

Just use the PHP command line interface (CLI) and specify the file name of the script to be executed as follows:

php script.php

7) How to run the interactive PHP shell from the command line interface?

Just use the PHP CLI program with the option -a as follows:

php -a

8) What is the correct and the most two common way to start and finish a PHP block of code?

The two most common ways to start and finish a PHP script are:

 <?php [   ---  PHP code---- ] ?> and <? [---  PHP code  ---] ?>

9) How can we display the output directly to the browser?

To be able to display the output directly to the browser, we have to use the special tags <?= and ?>.

10) What is the main difference between PHP 4 and PHP 5?

PHP 5 presents many additional OOP (Object Oriented Programming) features.

11) Is multiple inheritance supported in PHP?

PHP supports only single inheritance; it means that a class can be extended from only one single class using the keyword ‘extended’.

12) What is the meaning of a final class and a final method?

‘final’ is introduced in PHP5. Final class means that this class cannot be extended and a final method cannot be overridden.

13) How is the comparison of objects done in PHP?

We use the operator ‘==’ to test is two objects are instanced from the same class and have same attributes and equal values. We can test if two objects are referring to the same instance of the same class by the use of the identity operator ‘===’.

14) How can PHP and HTML interact?

It is possible to generate HTML through PHP scripts, and it is possible to pass pieces of information from HTML to PHP.

15) What type of operation is needed when passing values through a form or an URL?

If we would like to pass values through a form or an URL, then we need to encode and to decode them using htmlspecialchars() and urlencode().

16) How can PHP and Javascript interact?

PHP and Javascript cannot directly interact since PHP is a server side language and Javascript is a client-side language. However, we can exchange variables since PHP can generate Javascript code to be executed by the browser and it is possible to pass specific variables back to PHP via the URL.

17) What is needed to be able to use image function?

GD library is needed to execute image functions.

18) What is the use of the function ‘imagetypes()’?

imagetypes() gives the image format and types supported by the current version of GD-PHP.

19) What are the functions to be used to get the image’s properties (size, width, and height)?

The functions are getimagesize() for size, imagesx() for width and imagesy() for height.

20) How failures in execution are handled with include() and require() functions?

If the function require() cannot access the file then it ends with a fatal error. However, the include() function gives a warning, and the PHP script continues to execute.

21) What is the main difference between require() and require_once()?

require(), and require_once() perform the same task except that the second function checks if the PHP script is already included or not before executing it.

(same for include_once() and include())

22) How can I display text with a PHP script?

Two methods are possible:

<!--?php echo "Method 1"; print "Method 2"; ?-->

23) How can we display information of a variable and readable by a human with PHP?

To be able to display a human-readable result we use print_r().

24) How is it possible to set an infinite execution time for PHP script?

The set_time_limit(0) added at the beginning of a script sets to infinite the time of execution to not have the PHP error ‘maximum execution time exceeded.’ It is also possible to specify this in the php.ini file.

25) What does the PHP error ‘Parse error in PHP – unexpected T_variable at line x’ means?

This is a PHP syntax error expressing that a mistake at the line x stops parsing and executing the program.

26) What should we do to be able to export data into an Excel file?

The most common and used way is to get data into a format supported by Excel. For example, it is possible to write a .csv file, to choose for example comma as a separator between fields and then to open the file with Excel.

27) What is the function file_get_contents() useful for?

file_get_contents() lets reading a file and storing it in a string variable.

28) How can we connect to a MySQL database from a PHP script?

To be able to connect to a MySQL database, we must use mysqli_connect() function as follows:

<!--?php $database = mysqli_connect("HOST", "USER_NAME", "PASSWORD"); mysqli_select_db($database,"DATABASE_NAME"); ?-->

29) What is the function mysql_pconnect() useful for?

mysql_pconnect() ensure a persistent connection to the database, it means that the connection does not close when the PHP script ends.

This function is not supported in PHP 7.0 and above

30) How be the result set of Mysql handled in PHP?

The result set can be handled using mysqli_fetch_array, mysqli_fetch_assoc, mysqli_fetch_object or mysqli_fetch_row.

31) How is it possible to know the number of rows returned in the result set?

The function mysqli_num_rows() returns the number of rows in a result set.

32) Which function gives us the number of affected entries by a query?

mysqli_affected_rows() return the number of entries affected by an SQL query.

33) What is the difference between mysqli_fetch_object() and mysqli_fetch_array()?

The mysqli_fetch_object() function collects the first single matching record where mysqli_fetch_array() collects all matching records from the table in an array.

34) How can we access the data sent through the URL with the GET method?

To access the data sent via the GET method, we use $_GET array like this:
$variable = $_GET["var"]; this will now contain 'value'

35) How can we access the data sent through the URL with the POST method?

To access the data sent this way, you use the $_POST array.

Imagine you have a form field called ‘var’ on the form when the user clicks submit to the post form, you can then access the value like this:


36) How can we check the value of a given variable is a number?

It is possible to use the dedicated function, is_numeric() to check whether it is a number or not.

37) How can we check the value of a given variable is alphanumeric?

It is possible to use the dedicated function, ctype_alnum to check whether it is an alphanumeric value or not.

38) How do I check if a given variable is empty?

If we want to check whether a variable has a value or not, it is possible to use the empty() function.

39) What does the unlink() function mean?

The unlink() function is dedicated for file system handling. It simply deletes the file given as entry.

40) What does the unset() function mean?

The unset() function is dedicated for variable management. It will make a variable undefined.

41) How do I escape data before storing it in the database?

The addslashes function enables us to escape data before storage into the database.

42) How is it possible to remove escape characters from a string?

The stripslashes function enables us to remove the escape characters before apostrophes in a string.

43) How can we automatically escape incoming data?

We have to enable the Magic quotes entry in the configuration file of PHP.

44) What does the function get_magic_quotes_gpc() means?

The function get_magic_quotes_gpc() tells us whether the magic quotes is switched on or no.

45) Is it possible to remove the HTML tags from data?

The strip_tags() function enables us to clean a string from the HTML tags.

46) what is the static variable in function useful for?

A static variable is defined within a function only the first time, and its value can be modified during function calls as follows:

<!--?php function testFunction() { static $testVariable = 1; echo $testVariable; $testVariable++; } testFunction();        //1 testFunction();        //2 testFunction();        //3 ?-->

47) How can we define a variable accessible in functions of a PHP script?

This feature is possible using the global keyword.

48) How is it possible to return a value from a function?

A function returns a value using the instruction ‘return $value;’.

49) What is the most convenient hashing method to be used to hash passwords?

It is preferable to use crypt() which natively supports several hashing algorithms or the function hash() which supports more variants than crypt() rather than using the common hashing algorithms such as md5, sha1 or sha256 because they are conceived to be fast. Hence, hashing passwords with these algorithms can create vulnerability.

50) Which cryptographic extension provide generation and verification of digital signatures?

The PHP-OpenSSL extension provides several cryptographic operations including generation and verification of digital signatures.

51) How is a constant defined in a PHP script?

The define() directive lets us defining a constant as follows:

define ("ACONSTANT", 123);

52) How can you pass a variable by reference?

To be able to pass a variable by reference, we use an ampersand in front of it, as follows $var1 = &$var2

53) Will a comparison of an integer 12 and a string “13” work in PHP?

“13” and 12 can be compared in PHP since it casts everything to the integer type.

54) When is a conditional statement ended with endif?

When the original if was followed by: and then the code block without braces.

55) How is the ternary conditional operator used in PHP?

It is composed of three expressions: a condition, and two operands describing what instruction should be performed when the specified condition is true or false as follows:

Expression_1?Expression_2 : Expression_3;

56) What is the function func_num_args() used for?

The function func_num_args() is used to give the number of parameters passed into a function.

57) What does accessing a class via :: means?

:: is used to access static methods that do not require object initialization.

58) In PHP, objects are they passed by value or by reference?

In PHP, objects passed by value.

59) What’s the difference between __sleep and __wakeup?

__sleep returns the array of all the variables that need to be saved, while __wakeup retrieves them.

60) what is the definition of a session?

A session is a logical object enabling us to preserve temporary data across multiple PHP pages.

61) How to initiate a session in PHP?

The use of the function session_start() lets us activating a session.

62) How can you propagate a session id?

You can propagate a session id via cookies or URL parameters.

63) What is the meaning of a Persistent Cookie?

A persistent cookie is permanently stored in a cookie file on the browser’s computer. By default, cookies are temporary and are erased if we close the browser.

64) When do sessions end?

Sessions automatically end when the PHP script finishes executing but can be manually ended using the session_write_close().

65) What is the difference between session_unregister() and session_unset()?

The session_unregister() function unregister a global variable from the current session and the session_unset() function frees all session variables.

66) What does $_FILES means?

$_FILES is an associative array composed of items sent to the current script via the HTTP POST method.

67) What is the difference between $_FILES[‘userfile’][‘name’] and $_FILES[‘userfile’][‘tmp_name’]?

$_FILES[‘userfile’][‘name’] represents the original name of the file on the client machine,

$_FILES[‘userfile’][‘tmp_name’] represents the temporary filename of the file stored on the server.

68) How can we get the error when there is a problem to upload a file?

$_FILES[‘userfile’][‘error’] contains the error code associated with the uploaded file.

69) How can we change the maximum size of the files to be uploaded?

We can change the maximum size of files to be uploaded by changing upload_max_filesize in php.ini..

70) What does $_COOKIE mean?

$_COOKIE is an associative array of variables sent to the current PHP script using the HTTP Cookies.

71) What does the scope of variables mean?

The scope of a variable is the context within which it is defined. For the most part, all PHP variables only have a single scope. This single scope spans included and required files as well.

72) What are the two main string operators?

The first is the concatenation operator (‘.’), which returns the concatenation of its right and left arguments. The second is (‘.=’), which appends the argument on the right to the argument on the left.

73) What does the array operator ‘===’ means?

$a === $b TRUE if $a and $b have the same key/value pairs in the same order and of the same types.

74) How can we determine whether a PHP variable is an instantiated object of a certain class?

To be able to verify whether a PHP variable is an instantiated object of a certain class we use instanceof.

75) Is it possible to use COM component in PHP?

Yes, it’s possible to integrate (Distributed) Component Object Model components ((D)COM) in PHP scripts which is provided as a framework.