Dynamic logical expression parsing/evaluation in PHP?

Question

I have a need to evaluate user-defined logical expressions of arbitrary complexity on some PHP pages. Assuming that form fields are the primary variables, it would need to:

Answer 1

Much time has gone by since this question was asked, and I happened to be looking for an expression parser for php. I chose to use the ExpressionLanguage component from Symfony 2.4. It can be installed with no dependencies from composer via packagist.

composer require symfony/expression-language