The following is a listing of predefined identifiers in PHP. None of the identifiers listed here should be used as identifiers in a your scripts. These lists include keywords and predefined variable, constant, and class names. These lists are neither exhaustive or complete.
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, or function names. Using them as variable names is generally OK, but could lead to confusion.
Table G-1. PHP Keywords