So.....I have some large (LARGE) legacy sites on a IIS 6 server using PHP 4.4.1. Should I update? Yes, but I don't have the time to go through everything right now to fix other people's sloppy scripting and other issues that would make PHP 5 a no go on the server. Nonetheless, I have some new stuff to create and install, and I'd like to take advantage of PHP 5 (specifically for curl_multi_exec ). Is it possible to have 2 versions of PHP installed, where 4.4.1 or whatever is what IIS references, but I have then PHP 5 installed without any server connection -- just to run scripts locally on the server? Or will that inevitably cause problems?