Well, the easiest way would be to use system() or backticks to just execute the script of your choice with the proper interpreter.
That won't be elegant, fast, or particularly secure, but it will work.
There is a way to run perl code from PHP... (or ASP) using COM. It is a product created by ActiveState called PerlCOM that basically lets you create a COM object, feed it a string of Perl code, and it returns you what Perl returns. You can also set up a COM object to have methods which are really Perl functions. You call these functions just like COM methods.
If that is too expensive for you, you could try writing a custom PHP module which allows the execute of Perl code. Since Perl gives us ways to execute code through C, this shouldn't be too terribly difficult for simple projects.
As for running PHP from Perl, nothing comes to mind. I'm not very familiar with the Zend API, but it might be just a matter of writing a Perl module in C to interface with PHP.
Good luck..