[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/php5/callback_runme.php

@@ -0,0 +1,9 @@
+<?php
+
+require "tests.php";
+require "callback.php";
+// In 2.0.6 and earlier, the constant was misnamed.
+if (gettype(callback::FOO_I_Cb_Ptr) !== 'resource') die("callback::FOO_I_Cb_Ptr not a resource\n");
+
+check::done();
+?>