[PHP] How to convert a string into a mathematical expression?

[Christodoulou Demetris]

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.