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
Strictly speaking, false, nonnull, null, and 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 echo and invariant).