I just did some quick searching and came up empty. If you go with a dedicated (managed) server, you can usually ask them to install pretty much any version of PHP you want (you'll pay for any extra maintenance it requires, though). If you go with a shared server, you'll have a heck of a time finding a PHP 5 server that you can trust. Why are you keen to go with an OS X server? Why not Linux or FreeBSD? You pay a hefty, hefty premium for serving on OS X.