When interviewing for a position as a PHP programmer, you’ll undoubtedly encounter questions about your experience and knowledge of PHP. But this is a broad language, and there are a lot of details to know and remember when facing PHP interview questions. You can brush up before an interview by looking at some of the most common ones — and further exploring anything that doesn’t sound familiar. 

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PHP interview questions: environment, background, and usage

These are general questions that you might get about the language and what it’s used for. This is information that anyone who is a little familiar with PHP as a concept should know.

1. What does PHP stand for?

PHP stands for PHP Hypertext Preprocessor. This is a recursive acronym, somewhat like GNU (Gnu’s Not Unix). It’s important to understand this because it also defines the language. PHP is not a compiled language; it’s an interpreted language — it’s a “hypertext preprocessor.” It processes a website before the website gets processed in HTML, CSS, and JavaScript. This adds another layer onto the interpretation of the site.

2. What type of language is PHP?

PHP is an open-source scripting language. Specifically, it is not a compiled language but an interpreted language. PHP is closer to Perl and CGI than it is to C or Java. This is important to know because it changes the framework of how PHP is used. PHP is not compiled as an executable, but rather the script gets executed as it’s read, much like JavaScript.

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3. What is a LAMP stack?

LAMP is a very common environment for PHP. It refers to a Linux operating environment running an Apache server with MySQL and PHP installed (LAMP). One of the first things you need to run PHP is a server, and the Apache server environment is one of the most common. You can install an entire LAMP stack to start programming in PHP on your computer. 

4. Who created PHP and when?

Rasmus Lerdorf created PHP back in 1994. This is an important question not because of the specifics (you probably don’t really need to know the name Rasmus) but because of when. PHP is an old language. It’s evolved significantly since then, but it still has a lot of antiquated hallmarks, such as being able to execute in a runtime environment despite warnings and having (compared to other languages) limited object-oriented functionality.

5. What do you use PHP for?

You use PHP for back-end programming for websites. It can store data in a database, analyze data, and dynamically create HTML, JavaScript, and CSS. PHP can create extremely dynamic web applications and, when used with JavaScript, can create progressive web applications. 

6. What are some of the most popular sites built on PHP?

Perhaps the most popular site built in PHP is WordPress. WordPress is built from a PHP/MySQL backend, which means that a large percentage of the websites on the internet are built on PHP. Other famous systems designed in PHP include Joomla, Magneto, and Drupal. These are all content management systems (CMS) that are designed to make it easier for others to build websites.

7. What programming languages are most similar to PHP?

Perl and C are the programming languages most similar to PHP. But while PHP has remained popular, Perl and C are now considered to be mostly defunct. PHP remains a popular language that’s still in use today.

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8. What is a PHP function?

A PHP function is a block of code you can reuse without having to repeat the code. For instance, you might have a PHP function that prints out header information:

function header() {
		print("<header><title>My PHP Script</title></header>");
	}

Whenever you wanted to print out a header, you would just type:

	header();

If you needed to change the header text, you would only need to change the function, rather than the text on every page.

9. What extensions does PHP use?

Typically, you would use a .PHP, .PHTML, or .PHP5 extension with PHP, with .PHP being the most common. However, by configuring PHP you can actually use any extension with PHP. Some websites use a .HTML extension with PHP; the server knows to process the files through PHP regardless.

10. What terminates lines in PHP?

As with many languages, a semi-colon is what terminates a line in PHP:

	echo "Hello, World!";

Without a semi-colon, an error will be thrown.

11. How do you create a comment in PHP?

You create comments in PHP with // for a single line comment and /* */ for a multi-line comment:

//This comment is on a single line.
	/* This comment
		Spans multiple
			Lines */

As with many other types of code, a PHP script should be commented on as thoroughly as possible to explain the code and what it does.

12. Is PHP case sensitive or case insensitive?

Yes and no. PHP is case sensitive insofar as variable names are case sensitive. If you have a variable called strHelloWorld, you need to reference it as strHelloWorld, not strhelloworld or STRHELLOWORLD. But function names, even user-named functions, are not case sensitive. This can confuse programmers when they first start.

13. How do you insert PHP scripts into a page? 

Adding PHP scripts into an HTML page is done as such:

<?PHP
		//PHP code goes here
	?>

Everything around the PHP script above can be HTML. 

14. Are PHP web developers front-end or back-end developers?

PHP developers are back-end developers. HTML, CSS, and JavaScript are all front-end technologies. PHP and MySQL are back-end technologies. But there are full stack developers who will learn HTML, CSS, JavaScript, PHP, and MySQL.

15. Can a PHP program run standalone?

PHP programs are not designed to run standalone. Rather, a PHP program is an interpreted language. PHP has to be installed on a web server for it to be interpreted. Unlike JavaScript, PHP is interpreted on the server-side rather than the client-side. That means that the script will interpret the same regardless of web browser, but it also means the program needs the server to run.

16. What is a session in PHP?

A PHP session simply stores user-related data on the server rather than with the user. PHP sessions and PHP session IDs are frequently used when users log into a website. The PHP site identifies the user on the back-end and serves the user-customized content (such as their account information).

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17. How do you create a PHP function?

PHP doesn’t have a lot of controls around its function declarations. You can create a function as such:

	function HelloWorld() {
		print("Hello, world!");
	}

As you can see, you don’t need to type the function or anything — the function can stand alone.

18. How does PHP interact with HTML?

PHP is inserted into HTML. A .PHP, .PHTML, or .PHP5 page is a page that is being processed through PHP before it is being processed as HTML. PHP code will generate the HTML that is then analyzed by the browser. So, the PHP code is initiated first, and then the HTML, CSS, and JavaScript code

HTML does not interact with PHP. HTML can form a wrapper around PHP, but the HTML code itself does nothing to the PHP code. So, the interaction between PHP is asynchronous; it moves one way. 

19. Is PHP strictly typed?

No. PHP is not a strictly typed language. You don’t need to tell the PHP processor what type of variable you’re creating when you construct variables. You would just create a variable like so:

	$helloWorld = "Hello World.";

PHP knows that the variable is a string in the above code only because you have given it a string. If you had instead created a variable as such:

	$helloWorldBool = 1;

PHP would know that it was an integer.

20. How would you create a FOR() loop in PHP?

A FOR() loop in PHP is a loop that will iterate through a block of code a specific number of times. One writes it like this:

	for($int = 1; $int <= 10; $int++) {
		print("$int<br>");
	}

The above code will print the numbers 1 through 10. Note that we used an HTML <br> in the code to send a “break” to the browser. 

21. How would you create a WHILE() loop in PHP?

A WHILE() loop in PHP is a loop that will continue going until meeting a certain condition. You write it like this:

	$int = 1;
	while($int <= 10) {
		print("$int<br>");
		$int = $int+1;
	}

This does the same thing the earlier FOR() loop did; it prints the numbers 1 through 10. Traditionally, you don’t use a WHILE() loop to iterate between multiple steps (because that’s what a FOR() loop is for).

In languages like C, C++, and C#, a WHILE() loop might do something like collect information from a user. While the user had not responded to a prompt, a collection prompt would be provided.

But in PHP, the WHILE() loop is often avoided. An issue with the WHILE() loop is that it can potentially represent a completely recursive structure; in essence, it may never be escaped. It is easier to create a WHILE() loop that’s recursive than a FOR() loop that’s recursive because a WHILE() loop may never meet its criteria.

22. How would you create a SWITCH case in PHP?

A SWITCH() case prints a completely different block of code depending on what case you give it. It’s written like this:

	$int = rand(1,4);
	switch($int) {
		case 1: echo "You win!"; break;
		case 2: echo "You lost. But close!"; break;
		case 3: echo "You lost. Considerably!"; break;
		case 4: echo "You win!"; break;
	}

The above code creates a random number between 1 through 4. If it’s 1 or 4, it prints that you won. But if it’s 2 or 3, it prints that you’ve lost. 

Note that every case ends with “break.” We could have written this code like this:

	$int = rand(1,4);
	switch($int) { 
		Case 1: 
		Case 4: echo "You win!"; break;
		Case 2: 
		Case 3: echo "You lost!"; break;
	}

This highlights a couple of interesting things about “case.” First, the cases don’t need to be in order. Second, the cases will flow through to the next case until it hits a break.

In many situations, “case” syntax is considered deprecated because it is such an uncontrolled syntax. You could, instead, write this as an IF/THEN function:

	$int = rand(1,4);
	if($int == 1 || $int == 4) {
		print("You won!");
	} else {
		print("You lost!");
	}

Generally, the IF/THEN syntax is better controlled. But there are still situations where a SWITCH() case may be the best option.

23. How would you open a database session in PHP?

You can open a database session in PHP by using the mysqli_connect() function.

	mysqli_connect();

At the end, you would need to use the mysqli_close(); function to close the database.

While you’re manipulating the database, you will use mysqli_query(); functions. 

24. What is the PHP.ini file?

The PHP.INI file stores the PHP configuration information. You would want to update this file if anything about your configuration changes, such as the location of your PHP installation.

If you ever incorrectly configure your PHP file, you can delete it. PHP will create a new PHP.INI file. You can also rename it to something like “PHP.INI.BAK” so you can restore your old configuration if necessary.

25. What is the difference between a POST and a GET?

POST will invisibly send information, such as form information, to the server. GET will send the information as a formatted URL. In general, you should use POST rather than GET unless you want the user to be able to copy and paste the URL for later. 

26. What does the include function do?

The PHP include function includes a file. You can use it to combine multiple PHP files, such as a header file, body file, and footer file.

When a PHP script includes a file, it’s essentially like copying and pasting the contents of that file directly into the code:

		INCLUDE("file.txt");

You can also use the REQUIRE command to include a file and require its inclusion:

		REQUIRE("file.txt");

For the most part, you should use the INCLUDE_ONCE function rather than the INCLUDE function. Otherwise, it is possible to accidentally include a file multiple times (or even infinite times, if there’s recursion).

27. How would you find out what version of PHP you have?

You can find out what version of PHP you have and what your version of PHP supports by running the phpinfo() function within a PHP script.

You should always try your best to upgrade to the newest version of PHP. Like other open-source solutions, PHP gets updated and patched whenever vulnerabilities are discovered. A PHP installation that has not been updated in some time may have vulnerabilities and security issues.

28. What are PHP notices, warnings, and fatal errors?

Notices are something you should not do, but it’s OK to do, such as using a deprecated function. A warning is something that isn’t operating correctly but is not a catastrophic failure. A fatal error is something that does fail catastrophically; it stops the script from running altogether.

When you upgrade your PHP, you will likely get numerous notices and warnings, but not fatal errors. You can suppress these errors by turning off debugging information in your PHP.INI file. In general, your live environment should always have debugging suppressed, whereas your production environment should always have it active.

29. What is a constant in PHP?

A constant is a global variable in PHP that never changes. It’s generally used to track things, such as whether you’re currently in error checking mode. You could use a toggle, for instance, to either display debugging data or not display debugging data. Constants are usually kept at the top of a script. 

Constants differ from global variables. Global variables are variables that are used throughout a program but can change. 

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30. How do you use image functions in PHP? 

Most functions in PHP are built-in. You can use image functions in PHP using the GD Library. The GD Library is a built-in library that’s included within PHP, but that isn’t used except for image-related functions.

Common PHP image functions include: getimagesize(), imagecreate(), and imagedestroy(). Image functions are one of the most challenging functions in PHP as PHP isn’t really designed for robust image manipulation.

31. Should you use include or require?

In general, you should always use require rather than include, although you should also know the differences. Include includes a file, while require both includes it and requires the inclusion for the script to run. It’s very rare that you don’t need an included file to be present to run a script. Other than that, include and require are used in the exact same way:

	include("filename.txt");	//Will run even if filename.txt isn't found.
	require("filename.txt");	//Will exit if filename.txt isn't found.

There are times when you may be including an optional script, but it’s rare. To maintain control over your system, it’s better to use require.

32. Should you use require or require_once?

It’s generally considered a best practice to use require_once. Require and require_once fulfill the same functions; they require an included file. But require_once makes sure that it’s only required_once, whereas require might run multiple times.

33. What does set_time_limit() do?

The set_time_limit() function tells the PHP processor exactly how long you want to allow for the script to run. You may need to set_time_limit(0) while testing a program that’s taking a long time to execute, for instance. In practice, you don’t want to abuse this; it can lead to very slow-loading websites.

34. What are some popular PHP frameworks?

CakePHP, CodeIgniter, Zend, Symfony, and Yii2 are all popular PHP frameworks. CakePHP is an MVC (model-view-controller) framework frequently used to launch PHP projects quickly. A developer should have some knowledge of at least one MVC framework, as most PHP projects will begin with a framework rather than from scratch.

35. True or false: does PHP include multiple inheritances?

False. PHP only supports single inheritance. While this can be inconvenient, it simplifies code. A class can extend from a single class using the keyword “extended.” From there, an extended class cannot extend again.

36. What’s the difference between == and === in PHP?

The == operator tells you if a value matches. The === operator tells you if a value and type matches. This can be an important part of error checking, but many people don’t actually use the === operator.

37. What is the difference between “echo” and “print?”

Echo is faster, has no return value, and can take multiple arguments. But effectively, echo and print work very similarly. Most developers prefer to use “print” because it looks more like a function, but echo is often used for debugging purposes.

To display Hello, World in echo:

	echo "Hello, World!";

To display Hello, World in print:

	print("Hello, World!");

You can see how PRINT() syntax looks more formal, but many developers use “echo” as a way to track their progress as they program. For instance:

	echo $debugVar;

This is a fast way to print a debugging variable quickly.

38. How do you stop PHP code?

You can stop your PHP code at any time using the commands die() or exit(). These commands are identical; you can use them interchangeably.

39. How can you execute a script from the command line?

If you’re using BASH, GIT, VisualBasic, or another command-line program, you can execute PHP script (rather than using a server) like so:

	PHP script.php

Make sure that you include not only the script name but also the extension, as PHP scripts can have varied extensions.

40. Why would you use AJAX with PHP?

AJAX can help you transfer variables and data between PHP and JavaScript. There are other ways to do this, but AJAX is the most common. Today, PHP is frequently used in tandem with HTML5, CSS, and JavaScript to create web applications and progressive web applications. Without being able to interact with JavaScript variables, this is difficult.

Studying for PHP interview questions in 2022

PHP interview questions aren’t always predictable. They’re meant to test your knowledge of the PHP language thoroughly. If you want to make sure you ace interviews in PHP in 2022, you should consider brushing up on the language itself. There are a number of courses, lessons, and bootcamps available that can help you learn more about PHP.

Page Last Updated: February 2022

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