Defines a named constant. See the section on constants for more
details.
The name of the constant is given by name; the value is given by value.
The optional third parameter
case_insensitive is also available. If the value TRUE is given, then the constant
will be defined case-insensitive. The default behaviour is
case-sensitive; i.e. CONSTANT and Constant represent
different values.
成功回傳TRUE失敗回傳FALSE。
See also defined(),
constant() and the section on Constants.