I wonder why there's no document that maps major format IDs to corresponding PHP and Zend version.
Sometimes it is just a must to support some old applications that are based on ID=1 or ID=2 of Zend encoding. And it's heavy time for a newcomer to sort out what versions of PHP & Zend loader are required.
PS: Besides, it's a shame that there's no backward compatibility in Zend (using some bytecode-compiling wrappers or whatever). Even at a cost of speed / cpu load, this should be done due to security issues in older versions of PHP.