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PHP Keywords

By Shahnail Khan Last updated : December 7, 2023

Like any programming language, PHP has its set of reserved words known as keywords. In this tutorial, we'll explore PHP keywords, including their definition, and syntax.

PHP Keywords

Keywords have specific meanings in the language and cannot be used for naming variables, functions, or classes.

PHP Keywords List

Keyword	Description	Syntax
abstract	Used to declare an abstract class or method	abstract class Shape { abstract function draw(); }
and	Logical AND operator	if ($age >= 18 and $hasID) { ... }
array	Used to declare an array	$fruits = array("apple", "banana", "orange");
as	Used to alias a class or function	use DateTime as MyDate;
break	Exits a loop or switch statement	for ($i = 0; $i < 10; $i++) { if ($i === 5) { break; } }
callable	Represents a callable entity (function or method)	$callback = function($a, $b) { return $a + $b; };
case	Used in switch statements	switch ($day) { case "Monday": ...; case "Tuesday": ...; }
catch	Exception handling block	try { ... } catch (Exception $e) { ... }
class	Used to declare a class	class User { public $name; public function __construct($name) { $this->name = $name; } }
clone	Creates a copy of an object	$user2 = clone $user1;
const	Used to define a constant	const PI = 3.14159265359;
continue	Skips the current iteration of a loop	for ($i = 0; $i < 10; $i++) { if ($i === 5) { continue; } ... }
declare	Declares variables with specific types	declare(strict_types=1);
default	Used in switch statements	switch ($day) { case "Monday": ...; default: ...; }
die	Prints a message and exits the script	die("Error!");
do	Used with while loop to execute code at least once	do { ... } while ($condition);
echo	Outputs a string	echo "Hello, World!";
else	Used with if statements	if ($age >= 18) { ... } else { ... }
elseif	Used with if statements as a shortcut for "else if"	if ($age >= 21) { ... } elseif ($age >= 18) { ... } else { ... }
empty	Checks if a variable is empty	if (empty($name)) { ... }
enddeclare	Used to close a declare block	enddeclare();
endfor	Ends a for loop	for ($i = 0; $i < 10; $i++) { ... } endfor;
endforeach	Ends a foreach loop	foreach ($fruits as $fruit) { ... } endforeach;
endif	Ends an if statement	if ($age >= 18) { ... } endif;
endswitch	Ends a switch statement	switch ($day) { ... } endswitch;
endwhile	Ends a while loop	while ($condition) { ... } endwhile;
eval	Evaluates a string as code	eval('$result = 1 + 2;');
exit	Prints a message and exits the script	exit("Error!");
extends	Used to inherit properties and methods from another class	class User extends Person { ... }
final	Prevents a class from being inherited	final class Logger { ... }
finally	Always executes after a try/catch block, regardless of whether an exception was thrown	try { ... } catch (Exception $e) { ... } finally { ... }
for	Looping structure	for ($i = 0; $i < 10; $i++) { ... }
foreach	Looping structure for traversing arrays	foreach ($fruits as $fruit) { ... }
function	Used to define a function	function sum($a, $b) { return $a + $b; }
global	Used to declare variables accessible within the current scope	global $name;
goto	Jump to a specific label within the script	goto myLabel;
if	Conditional statement	if ($age >= 18) { ... }
implements	Used in a class to implement an interface	class MyClass implements MyInterface { ... }
include	Includes and evaluates a specified file	include "filename.php";
include_once	Includes and evaluates a file only once	include_once "filename.php";
instanceof	Checks if an object is an instance of a class	if ($obj instanceof MyClass) { ... }
insteadof	Used in trait to specify a precedence of a certain trait over another	use Trait1, Trait2 { Trait1::method insteadof Trait2; }
interface	Declares an interface	interface MyInterface { ... }
isset	Checks if a variable is set	if (isset($variable)) { ... }
list	Assigns variables as if they were an array	list($a, $b) = [1, 2];
namespace	Declares a namespace	namespace MyNamespace;
new	Creates a new object	$object = new ClassName();
or	Logical OR operator	if ($condition1 or $condition2) { ... }
print	Outputs a string	print "Hello, World!";
private	Specifies that a method or property can only be accessed within the class	private $name;
protected	Specifies that a method or property can only be accessed within the class and its subclasses	protected $name;
public	Specifies that a method or property can be accessed from outside the class	public $name;
require	Requires and evaluates a specified file	require "filename.php";
require_once	Requires and evaluates a file only once	require_once "filename.php";
return	Exits a function and returns a value	function add($a, $b) { return $a + $b; }
static	Declares a static method or property	static $counter = 0;
switch	Selects one of many blocks to be executed	switch ($day) { case "Monday": ...; break; }
throw	Throws an exception	throw new Exception("Error message");
trait	Introduces a trait into a class	class MyClass { use MyTrait; }
try	Implements exception handling	try { ... } catch (Exception $e) { ... }
unset	Unsets a variable	unset($variable);
use	Imports a namespace or trait	use MyNamespace\MyClass;
var	Declares a variable	var $name;
while	Creates a while loop	while ($condition) { ... }

PHP Keywords Examples

Explore these examples to understand about the keywords in PHP scripts.

To understand both of the examples, you should have the knowledge of the following PHP topics:

• PHP echo statement
• PHP conditional statements
• PHP exception handling

Example 1

Create a PHP function called divideNumbers that takes two parameters and returns the result of dividing them. Handle division by zero using a try-catch block and throw a custom exception. (Learn: Exception Handling in PHP)

<?php

function divideNumbers($numerator, $denominator)
{
    try {
        if ($denominator === 0) {
            throw new Exception("Cannot divide by zero");
        }

        return $numerator / $denominator;
    } catch (Exception $e) {
        echo "Error: " . $e->getMessage();
    }
}

$result = divideNumbers(10, 0);

?>

The output of the above example is:

Error: Cannot divide by zero

In this example, we have a function called "divideNumbers" that divides two numbers. The tricky part is, if the second number (denominator) is zero, it can cause an error. So, we use the keywords try, catch, and throw to handle this situation. When the denominator is zero, the program doesn't stop executing; instead, it throws a special message saying "Cannot divide by zero," and the program continues running without any major problems.

Example 2

You are asked to implement a simple arithmetic calculator in PHP. Write a function "calculate" that takes two numbers and an arithmetic operator as parameters and returns the result of the operation.

• The function should handle addition (+), subtraction (-), multiplication (*), and division (/) operations.
• For division, return null if the second number is zero.
• If the operator is not one of the valid arithmetic operators, return null.

Solution

Read the problem carefully. Once you read, think of the easiest approach. Since we have to perform three tasks, we can use the switch statement.

<?php
function calculate($num1, $num2, $operator)
{
    switch ($operator) {
        case "+":
            return $num1 + $num2;
        case "-":
            return $num1 - $num2;
        case "*":
            return $num1 * $num2;
        case "/":
            // Check for division by zero
            return $num2 != 0 ? $num1 / $num2 : null;
        default:
            return null; // Invalid operator
    }
}

// Test the function with the provided examples
$result1 = calculate(5, 3, "+");
$result2 = calculate(10, 4, "-");
$result3 = calculate(6, 2, "*");
$result4 = calculate(8, 2, "/");
$result5 = calculate(8, 2, "%");

echo $result1 . "<br>";
echo $result2 . "<br>";
echo $result3 . "<br>";
echo $result4 . "<br>";
var_dump($result5);

?>

The output of the above example is:

8
6
12
4
NULL

This solution uses a switch statement to perform the appropriate arithmetic operation based on the provided operator. The function returns the result of the operation or null for invalid operators or division by zero.