以下是 PHP 中预定义标识符的列表。没有列在下面的标识符可以在脚本中作为自定义标识符使用。这些列表包括关键字和预定义变量、常量和类名。这些列表可能有遗漏，它是不完整的。
SR (2007-10-04 10:53:49)
[Editorial note: parent and self are reserved class names. Functions and constants with their names can be declared, but not userspace classes]
true, false and null are not listed because they are globally-defined constants, not reserved words.
No idea about parent or self, though.
londonx at gmail dot com (2007-03-05 02:14:38)
Naming a PHP5 class "variant" (with a constructor called the same) and then making an instance of it using __autoload triggers an error which disappears as soon as the name is changed. This should mean that "variant" is also a reserved word.
(2006-11-01 04:42:12)
Two things:
1. It would be nice for this list to differentiate those reserved words, which are language constructs used *with* parentheses (isset, unset, empty, eval, exit, die, ...) and those rather to be used *without* parentheses (all includes, echo, print, return, ...?)
This would really help define some common coding styles...
2. Do you write NULL, TRUE, FALSE or null, true, false? (I find CAPITAL letters better, however typing them can be annoying so I stick with using all lowercase letters, but what's the standard?)
(2006-01-10 03:41:27)
I don't see the boolean constants 'true' and 'false' listed among the reserved words/keywords/constants.