Source Code Fundamentals: Keywords
Keywords are case-sensitive, reserved words; they cannot be used as names.
abstract arraykey as async await break case catch class classname clone const continue default do dynamic echo else elseif enum extends final finally for foreach function if implements inout instanceof insteadof interface mixed namespace new newtype noreturn num parent private protected public require require_once return self shape static switch throw trait try tuple type use while yield
true are not keywords; however, they do have predefined meanings,
and can be thought of as keywords. Likewise, for the names of the intrinsic functions (such as