[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/php5/callback/runme.php

@@ -0,0 +1,47 @@
+<?php
+
+# This file illustrates the cross language polymorphism using directors.
+
+require("example.php");
+
+# Class, which overwrites Callback::run().
+
+class PhpCallback extends Callback {
+  function run() {
+    print "PhpCallback.run()\n";
+  }
+};
+
+# Create an Caller instance
+
+$caller = new Caller();
+
+# Add a simple C++ callback (caller owns the callback, so
+# we disown it first by clearing the .thisown flag).
+
+print "Adding and calling a normal C++ callback\n";
+print "----------------------------------------\n";
+
+$callback = new Callback();
+$callback->thisown = 0;
+$caller->setCallback($callback);
+$caller->call();
+$caller->delCallback();
+
+print "\n";
+print "Adding and calling a PHP callback\n";
+print "------------------------------------\n";
+
+# Add a PHP callback.
+
+$callback = new PhpCallback();
+$callback->thisown = 0;
+$caller->setCallback($callback);
+$caller->call();
+$caller->delCallback();
+
+# All done.
+
+print "php exit\n";
+
+?>