[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

Examples/test-suite/php/director_thread_runme.php

@@ -3,7 +3,8 @@
 require "tests.php";
 require "director_thread.php";
 
-# Fails in a ZTS-build of PHP - see: https://github.com/swig/swig/pull/155
+# Fails in a ZTS-build of PHP5 - see: https://github.com/swig/swig/pull/155
+# FIXME: Does this still fail in a threaded build of PHP7?
 exit(0);
 
 // No new functions