Davey Shafik has a helpful hint for avoiding one of the worst functions to use in PHP - eval.

There are a shed-load of ways to "eval()" code without actually calling the eval() function '" usually done simply to avoid the use of the dreaded "evil()" function, but often times because the system has eval() disabled using "disable_functions" in php.ini. Here is another simple way to avoid eval() without writing out files to the filesystem

His example uses the streams wrapper to natively execute the code from a string variable as a data element, base64 decoded. It's more of a proof-of-concept than anything else, but its an interesting solution to a tough problem to solve at times.