PHP: Calculate a math function f(x) in a string

Question

Is it possible to calculate a math function f(x) that in a string. Something like this:

$function = '2x+3';
$x = 4;
math_function($function, $x); //Shoud produce 11

I can't find a library for tasks like this on PHP.net or with Google, but I don't think I am the first one that wants this?

Mark Baker · Accepted Answer · 2010-11-18 16:06:36Z

3

My standard answer to this question whenever it crops up:

Don't use eval (especially if the formula contains user input) or reinvent the wheel by writing your own formula parser.

Take a look at the evalMath class on PHPClasses. It should do everything that you want in a nice safe sandbox.

answered Nov 18, 2010 at 16:06

Mark Baker

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6 Comments

ircmaxell Over a year ago

+1... Building a math formula parser is a great academic problem (since there are many well defined edge-cases which yield to a non-trivial solution). But in terms of a practical problem, it's not worth the effort since there are tools built for it...

Woutb21 Over a year ago

Wow, that was quick! Thank you very much. I didn't even know the existence of eval()! I will take a look at it.

edorian Over a year ago

I'm confused: You say "Don't use eval" and then advice the use of a class that does exactly that. (Still perfectly valid answer that solves ops problem. So +1, even if the evalMath code makes me want to vomit ;) )

Mark Baker Over a year ago

@edorian - evalMath does NOT use eval! It uses Djikstra's shunting yard to parse the formula onto a stack, then processes operations using standard PHP methods in a controlled sandbox

Mark Baker Over a year ago

@edorian - that instance could also be replaced with $r = call_user_func($fnn,$op1);$stack->push($r); which is cleaner than using eval(), and that's not dissimilar to what I did with my own math parser.

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