Old version of PHP

Hello, I'm working on an ongoing project that depends on a shared webserver running an old version of PHP (4.1.2 I believe). Is there any good reason to stick with an older version of PHP, or might it be a valid suggestion to have it upgraded to something a bit more recent? I have a notion that upgrade attempts may have been snubbed out by the way things like register_globals are now handled by default, the perception being that newer versions of PHP are incompatible with older code. Mainly I'm concerned that code I test on our in-house server running version-current PHP will depend on function calls and language constructs that the production server's vintage PHP interpreter lacks. Some questions: Is there a way to get one instance of Apache to use different versions of PHP for various virtual hosts? Is there a way to analyze code for PHP version compliance? And finally, what is the earliest language version that can reliably handle a PEAR object? Thanks, -Dan