List of Reserved Words
PHP Manual

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. You cannot use any of the following words as constants, class names, function or method names. Using them as variable names is generally OK, but could lead to confusion.

As of PHP 7.0.0 these keywords are allowed as property, constant, and method names of classes, interfaces and traits, except that class may not be used as constant name.

PHP Keywords
__halt_compiler() abstract and array() as
break callable (as of PHP 5.4) 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 (as of PHP 5.5) for foreach function
global goto (as of PHP 5.3) if implements include
include_once instanceof insteadof (as of PHP 5.4) interface isset()
list() namespace (as of PHP 5.3) new or print
private protected public require require_once
return static switch throw trait (as of PHP 5.4)
try unset() use var while
xor yield (as of PHP 5.5)
Compile-time constants
__CLASS__ __DIR__ (as of PHP 5.3) __FILE__ __FUNCTION__ __LINE__ __METHOD__
__NAMESPACE__ (as of PHP 5.3) __TRAIT__ (as of PHP 5.4)

List of Reserved Words
PHP Manual