Top 20 PHP Interview Questions and Answers

We have added most frequently asked PHP Interview questions and answers for Phone Screening.

You can refer PHP site for Documentation and Installation.

Question 1: What’s the difference between the include() and require() functions?

Both include and require serve the same purpose, i.e. evaluate and include the requested file. The difference is that if an error occurs require will throw a fatal error and stop the execution of the script, while include will generate a warning and proceed with the execution of the code after.2

Question 2: What’s the difference between unset() and unlink()

Unset is used for destroying a variable(s), while unlink is used for deleting a file from the file system.

Question 3: What are references in PHP? What will the values of x and y be after executing the following code?

$x = '7';
$y = &$x;
$y = "3$x";

Question 4: What is the difference between errors and exceptions in PHP?

An exception can be thrown and is intended to be caught using a try-catch block. A PHP error on the other hand is non recoverable and can stop the whole execution of the program.

Question 5: What is the difference between GET and POST requests?

• GET appends the submitted data to the URL and is saved in the browser history, unlike POST which doesn’t show the submitted parameters in the URL and is never saved in history. That makes GET less secure and bad choice when dealing with sensitive data.
• GET has a length restriction, POST doesn’t.
• GET requests should only be used to retrieve data.

Question 6: Why would you use === instead of ==?

=== is used for strict comparison and checks both the value and the type of the operands. == on the other hand converts the operands to comparable types before checking, so it only takes the value into account. That makes === faster, because it doesn’t have to initiate a conversion operation.
Example: 1 == true will return true, while 1 === true will return false.

Question 7: What would 042 == 42 return?

The operation returns false.
A leading 0 indicates that the number is in the octal system, so the operation above means we are comparing an octal 42 with a decimal 42. That would translate to 34 == 42, which returns false.

Question 8: What’s the difference between array() and []?

They are the same. [] is a short syntax for array and acts as an alias. It’s introduced in PHP 5.4, so using it in any older version of PHP will fail with an error.

Question 9: What is the difference between single-quoted string ('text') and double- quoted string ("text")?

Using double quotes allows for parsing a variable inside the string, while that’s not possible with single quotes. Because of the additional operation of looking for a variable inside the string and getting its value, using single quotes is faster if you don’t need this added functionality.

$name = ‘John Doe’;
echo “I am $name”; /* prints I am John Doe */
echo ‘I am $name’; /* prints I am $name */

$x = true && false; 

/* $x is equal to false. 
Explanation: && executes first, so the operation on the right side of the = is calculated first (equals to false) and then assigned to $x. 
*/

$x = true AND false; 

/* $x is equal to true. Explanation: = has precedence over AND, so it executes first, meaning that $x is assigned to true. Then, the AND is executed in the form of $x AND false, which returns false but is not assigned to anything. 
*/

Question 11: What is the difference between echo and print in PHP?

echo doesn’t return anything, while print returns 1 to indicate the operation was successful. You can pass multiple strings to echo separated by a comma and it will print all of them. With print you can only send one parameter at a time.

Question 12: What will this function return?

$str = 'PHP is my language.';
if (strpos($str, 'PHP')) {
return true;} else {return false;}

This will return false. strpos looks for the position of ‘PHP’ inside the defined string. Since ‘PHP’ is at the beginning of the string, its position is 0. If ‘PHP’ didn’t exist in the string, it would return false.
Now, because the check is not done with strict comparison, this goes directly to the else and returns false. Doing it like if (strpos($str, 'PHP') === false) would fix the problem.

Question 13: When is the warning ‘Warning: Cannot modify header information – headers already sent’ triggered?

This warning is triggered when trying to modify the HTTP headers (by setting a session, cookie, response type, doing a redirect etc.) after already displaying something on the screen (echoing something, starting html, unnoticed white space before the php open tag, PHP error etc.).

Question 14: What are the __construct() and __destruct() methods in a PHP class?

The __construct() method is called immediately after a new instance of the class is being created and it’s used to initialize class properties. If the __construct() method is not implemented by the class itself, a built in constructor is called which creates the class instance. Similarly, the __destruct() method is called for destroying the created object when the script execution stops. The default built in destructor will be used if the class doesn’t implement its own.

Question 15: What are the 3 scope levels available in PHP and how would you define them?

• Private – Visible only in its own class
• Public – Visible to any other code accessing the class
• Protected – Visible only to classes parent(s) and classes that extend the current class

Question 16: What is the use of final keyword?

When a class method is defined as final (prefixing the method definition with the final keyword) it means that the child classes can not override that method.
If a class is defined as final, it means that it can not be extended.

Question 17: Does PHP support multi-inheritance?

PHP doesn’t support multi inheritance. Traits are introduced as a workaround for this in both PHP and other single inheritance languages.

Question 18: What are Traits?

Traits are a mechanism that allows code reuse in languages like PHP that don’t support multi inheritance. It is intended to group given functionality in a consistent way. It’s very similar to a class, but it can’t be instantiated on its own – it’s only used inside a given class to extend its functionality.

Question 19: What is the difference between an interface and an abstract class?

An interface specifies the methods that the class must implement, without defining how those methods are handled. A class implements an interface.
Abstract classes are classes that have at least one abstract method. They can’t be instantiated, only extended and all abstract methods have to be implemented in the child class. The class extends an abstract class.
Unlike with interfaces, with abstract classes you can have predefined methods that the child class can use.

Question 20: What is the difference between self and static keywords when used for calling static methods in a class?

When we use self in a class method, we reference the class that method was defined in. With static, we can reference the class that was called at runtime.

E.g.

class A {
public static function who() {echo __CLASS__;}public static function test() {self::who();}}
class B extends A {
public static function who() {echo __CLASS__;}}
B::test();

We use self to call the who method and no matter that we called the test method on B, the self keyword looks for the static method in the same class where the call is located (the parent class – A). So, the snippet above will output A.

Now, if we replace the test method in A with:

public static function test() {
static::who();}

B::test(); will output B. That’s because the static keyword doesn’t look into the class it’s located in, but the class the call was made on.21

Question 21: What is SQL Injection and how to prevent it?

SQL Injection is a technique where an attacker creates or alters existing SQL queries to execute dangerous commands on the database. This happens when the application is taking user input to build a database query and if there’s no proper validation on the input values, the attacker can mess up the whole application.
To prevent it, developers should always check and escape all user input before passing it to the database query.