Keywords in PHP

A keyword is a word reserved by the language for its core functionality—you cannot give a variable, function, class, or constant the same name as a keyword. Lists of keywords in PHP, which are case-insensitive.

and	$argc	$argv	as
break	case	cfunction	class
continue	declare	default	die
do	E_ALL	echo	E_ERROR
else	elseif	empty	enddeclare
endfor	endforeach	endif	endswitch
E_PARSE	eval	E_WARNING	exit
extends	FALSE	for	foreach
function	$HTTP_COOKIE_VARS	$HTTP_ENV_VARS	$HTTP_GET_VARS
$HTTP_POST_FILES	$HTTP_POST_VARS	$HTTP_SERVER_VARS	if
include	include_once	global	list
new	not	NULL	old_function
or	parent	PHP_OS	$PHP_SELF
PHP_VERSION	print	require	require_once
return	static	stdClass	switch
$this	TRUE	var	virtual
while	xor	_ _FILE_ _	_ _LINE_ _
_ _sleep	_ _wakeup	$_COOKIE	$_ENV
$_FILES	$_GET	$_POST	$_SERVER

In addition, you cannot use an identifier that is the same as a built-in PHP function.