Untrusted code execution in PHPMailer

Reported a vulnerability in PHPMailer where a function could run unexpectedly while sending a mail leading to untrusted code execution.

validateAddress function used to validate email addresses, uses call_user_func to call the callable from the name of callable provided to the function as an argument $patternselect. But if no argument is passed, the function sets "php" as default value to $patternselect variable on line 1337 (ironically). If a callable (function) named php is present in the scope, the function gets executed.