[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/funcptr/runme.php

@@ -0,0 +1,24 @@
+<?php
+
+require "example.php";
+
+$a = 37;
+$b = 42;
+
+# Now call our C function with a bunch of callbacks
+
+print "Trying some C callback functions\n";
+print "    a        = $a\n";
+print "    b        = $b\n";
+print "    ADD(a,b) = ". do_op($a,$b,ADD)."\n";
+print "    SUB(a,b) = ". do_op($a,$b,SUB)."\n";
+print "    MUL(a,b) = ". do_op($a,$b,MUL)."\n";
+
+print "Here is what the C callback function objects look like in php\n";
+print "Using swig style string pointers as we need them registered as constants\n";
+print "    ADD      = " . ADD . "\n";
+print "    SUB      = " . SUB . "\n";
+print "    MUL      = " . MUL . "\n";
+
+?>
+