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.