[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/example.i

@@ -0,0 +1,15 @@
+/* File : example.i */
+%module example
+%{
+#include "example.h"
+%}
+
+/* Wrap a function taking a pointer to a function */
+extern int  do_op(int a, int b, int (*op)(int, int));
+
+/* Now install a bunch of "ops" as constants */
+%constant int (*ADD)(int,int) = add;
+%constant int (*SUB)(int,int) = sub;
+%constant int (*MUL)(int,int) = mul;
+
+