Hello,
It is possible to run both the PHP5 versions in the server. But I do not thinks it is possible for a client to run the particular PHP5 version. If you install PHP4 and PHP5 version, it is possible to give client a choice.
B/w I thinks there are only minor changes in the 5.2 and 5.3 which includes the Zend issue. Is there any particular reason for compiling like that?