[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/constants/example.i

@@ -0,0 +1,26 @@
+/* File : example.i */
+%module example
+
+/* A few preprocessor macros */
+
+#define    ICONST      42
+#define    FCONST      2.1828
+#define    CCONST      'x'
+#define    CCONST2     '\n'
+#define    SCONST      "Hello World"
+#define    SCONST2     "\"Hello World\""
+
+/* This should work just fine */
+#define    EXPR        ICONST + 3*(FCONST)
+
+/* This shouldn't do anything */
+#define    EXTERN      extern
+
+/* Neither should this (BAR isn't defined) */
+#define    FOO         (ICONST + BAR)
+
+/* The following statements also produce constants */
+%constant int     iconst = 37;
+%constant double  fconst = 3.14;
+
+