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:

• substitute"varibles" for form fields values;
• handle comparison operators, minimally ==, <, <=, >= and > by symbol, name (eg eq, lt, le, ge, gt respectively);
• handle boolean operators not, and, or and possibly xor by name, symbol (eg !, &&, || and ^^ respectively);
• handle literal values for strings and numbers;
• be plaintext not XML (eg "firstname == '' or lastname == ''); and
• be reasonably performant.

Now in years gone by I've written recursive descent parsers that could build an expression tree and do this kind of thing but thats not a task I'm relishing in PHP so I'm hoping there are things out there that will at least get me some of the way there.

Suggestions?

Answer 1

Check create_function, it creates an anonymous function from the string parameters passed, I'm not sure about its performance, but it's very flexible...

Answer 2

If I understand the problem correctly, you want the users to write out functions in non-PHP, and then have PHP interpret it?

If so, you could simply take their string and replace "lt" with "<" and "gt" with ">" ... then do eval().

I have a hunch the problem isn't this simple, but if it is, eval() could do the job. Of course, then you're opening yourself up for any kind of attack.

Answer 3

Take a look at my infix to postfix example I think you could port it to PHP with relative ease. It only uses an array and some switches. No trees. A stack is only needed to run the postfix result.