[PHP] Add support for PHP7.

PHP5's C extension API has changed substantially so you need to use
-php7 to specify you want PHP7 compatible wrappers.
Fixes https://github.com/swig/swig/issues/571

Doc/Manual/SWIG.html

@@ -131,7 +131,8 @@ can be obtained by typing <tt>swig -help</tt> or <tt>swig
 -ocaml                Generate Ocaml wrappers
 -octave               Generate Octave wrappers
 -perl                 Generate Perl wrappers
--php                  Generate PHP wrappers
+-php5                 Generate PHP5 wrappers
+-php7                 Generate PHP7 wrappers
 -pike                 Generate Pike wrappers
 -python               Generate Python wrappers
 -r                    Generate R (aka GNU S) wrappers