Elijah Horton has a recent post to his site sharing a tool he's developed to sandbox and validate PHP code of user-contributed code.

Few quotes related to the PHP language are as pithy and resoundingly accurate as the phrase: "Eval is evil." The reasons are myriad: the eval() function basically gives whatever code is passed to it unlimited control of the parser, and this freedom makes eval() both a temptation for developers, who may need to dynamically control PHP at runtime, and a panacea for hackers who are ever-searching for more servers to add to their botnets. So, how does one make use of the extreme power available through runtime evaulation of PHP, without exposing one's server to near-certain rooting? Through a sandbox.

His tool - PHPSandbox, uses the PHP-Parser library to deconstruct the PHP code its given and look for issues. He gives an example of a call to mail and how it would catch the issue. He shows how to install it via Composer, how to configure it with whitelisted methods/functions. It also includes a way to overwrite function calls with a bit safer alternative.