How to make a calculator in PHP?

Question

I want to use PHP to calculate simple algebraic expressions like, 8*(5+1), entered via an tag by a normal user (which means,

Answer 1

I'd start by stripping the input of anything which shouldn't be in the expression (assuming you just want to allow add, subtract, multiply, divide, and no variables):

 $expr = preg_replace('/[^0-9+*\/-]/', '', $expr);

and then, once I'm confident nothing dangerous remains in the user input, simply pass the itthrough eval() to evaluate the expression:

 $result = eval("return $expr;");

No need to reinvent the wheel.

Edited to incorporate Kolink's corrections. Thanks!