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.
表格 G-1. PHP Keywords