How to convert a string into a mathematical expression?

Hi list. I have string variables that contain mathematical expressions. These strings are submitted through forms. For example if a user inserts the following-> 1+cos(0.3)*x into a text field with name expression in an HTML form then what i get on the server is the string variable $expression whose values is "1+cos(0.3)*x" Is there a way to convert the string variable, $expression, into a mathematical expression that can be calculated? Thanks in advance. DX.