The following is a listing of predefined identifiers in PHP. None
of the identifiers listed here should be used as identifiers in any of
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, function or method names.
Using them as variable names is generally OK, but could lead to confusion.