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List of Reserved Words

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The following is a listing of predefined identifiers in PHP. None of the identifiers listed here should be used as identifiers in any of your scripts unless explicitly noted otherwise. These lists include keywords and predefined variables, constant, and class names. These lists are neither exhaustive nor complete.

List of Keywords

These words have special meaning in PHP. Some of them represent things which look like functions, some look like constants, and so on - but they're not, really: they are language constructs. The following words cannot be used as constants, class names, or function names. They are, however, allowed as property, constant, and method names of classes, interfaces, traits and enums, except that class may not be used as constant name.

`__halt_compiler()`	abstract	and	`array()`	as
break	callable	case	catch	class
clone	const	continue	declare	default
`die()`	do	`echo()`	else	elseif
`empty()`	enddeclare	endfor	endforeach	endif
endswitch	endwhile	`eval()`	`exit()`	extends
final	finally	fn (as of PHP 7.4)	for	foreach
function	global	goto	if	implements
`include()`	`include_once()`	instanceof	insteadof	interface
`isset()`	`list()`	Match (as of PHP 8.0)	namespace	new
or	`print()`	private	protected	public
readonly (as of PHP 8.1.0) *	`require()`	`require_once()`	`return()`	static
switch	throw	trait	try	`unset()`
use	var	while	xor	yield
yield from

* readonly may be used as function name.

__CLASS__	__DIR__	__FILE__	__FUNCTION__	__LINE__
__METHOD__	__PROPERTY__	__NAMESPACE__	__TRAIT__

Predefined Classes

This section lists standard predefined classes. Miscellaneous extensions define other classes which are described in their reference.

Standard Defined Classes

These classes are defined in the standard set of functions included in the PHP build.

Directory: Created by dir().
stdClass: A generic empty class created as a result of typecasting to object or various standard functions.
__PHP_Incomplete_Class: Possibly created by unserialize().
Exception
ErrorException
php_user_filter
Closure: The predefined final class Closure is used for representing anonymous functions.
Generator: The predefined final class Generator is used for representing generators.
ArithmeticError
AssertionError
DivisionByZeroError
Error
Throwable
ParseError
TypeError

Special classes

Following identifiers may not be used as a class name as they have special purpose.

self: Current class.
static: Current class in runtime.
parent: Parent class.

Constants

List of other reserved words

The following words cannot be used to name a class, interface, trait or enum. Prior to PHP 8.0, they are also prohibited from being used in namespaces.

parent	self	int	float
bool	string	true	false
null	void (as of PHP 7.1)	iterable (as of PHP 7.1)	object (as of PHP 7.2)
mixed (as of PHP 8.0)	never (as of PHP 8.1)	array (as of PHP 8.5)	callable (as of PHP 8.5)

The following list of words have had soft reservations placed on them. Whilst they may still be used as class, interface, trait and enum names, usage of them is highly discouraged since they may be used in future versions of PHP.

enum

resource

numeric

Source: appendices/reserved.xml · from the official PHP manual (php/doc-en)