[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.c

@@ -0,0 +1,17 @@
+/* File : example.c */
+
+int do_op(int a, int b, int (*op)(int,int)) {
+  return (*op)(a,b);
+}
+
+int add(int a, int b) {
+  return a+b;
+}
+
+int sub(int a, int b) {
+  return a-b;
+}
+
+int mul(int a, int b) {
+  return a*b;
+}