Evaluating math without eval()

Hi all First post to this list! I'm trying to figure out how to evaluate a string with a mathematical expression and get a result, but without using eval() as I'm accepting user input into the string. I don't just want addition, subtraction, multiplication and division - I'd like to take advantage of other functions like ceil, floor etc. So the string "18-10" should give me 8, "ceil(1/2)*10" should be 10 (if my maths is correct) and the string "18-10;\r\nunlink('/var/www/*');" should not execute. Any ideas? Thanks! Jason