[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/callback/example.h

@@ -0,0 +1,22 @@
+/* File : example.h */
+
+#include <iostream>
+
+class Callback {
+public:
+	virtual ~Callback() { std::cout << "Callback::~Callback()" << std:: endl; }
+	virtual void run() { std::cout << "Callback::run()" << std::endl; }
+};
+
+
+class Caller {
+private:
+	Callback *_callback;
+public:
+	Caller(): _callback(0) {}
+	~Caller() { delCallback(); }
+	void delCallback() { delete _callback; _callback = 0; }
+	void setCallback(Callback *cb) { delCallback(); _callback = cb; }
+	void call() { if (_callback) _callback->run(); }
+};
+