[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/simple/example.i

@@ -0,0 +1,8 @@
+/* File : example.i */
+%module example
+
+%inline %{
+extern int    gcd(int x, int y);
+extern double Foo;
+void print_Foo();
+%}