You can file this one under the "bygone days of PHP/FI" category, but Adam Trachtenburg has posted about the old_function hack he uncovered.

I was reviewing Upgrading to PHP 5 today and I came across this lovely section in Appendix B, which I had completely forgotten about.

I must say, I used PHP/FI, but I never resorted to the old_function hack to ease my PHP 3 migration issues.

If you'd like to check out an old manual entry for this one, look here. It allowed you to declare a function using the syntax for PHP/FI2 even though you were in PHP 3.