using variable as math operator

[Muddy_Funster]

Curious...If I store values and operators in a DB table and want to use them to perform calculations, how would I go about doing that? eg.

Table has an "operator" column with |* 1.5|/ 0.75|+ 1.2| and I want to do something like desired_value = set_value affected by operator and value from table column, so if set_value = 10 I would want

desired_value = 10 * 1.5

desired_value = 10 / 0.75

desired_value = 10 + 1.2

 

Anything like that possible?

[trq]

eval.

[Muddy_Funster]

when I tried the following 3 options :

$a = '* 1.2';
$b = '10';
$c = eval("$a ".$b);
//$c = eval("$a $b");
//$c = eval($a ." $b");
echo $c;

I got

Parse error: syntax error, unexpected '*' in C:\xampp\htdocs\z.php(4) : eval()'d code on line 1

Does it not like math operators or am I doing it wrong?

[trq]

What your evaluating needs to make sense.

 

Your trying to execute * 1.210

 

What exactly do you expect to happen?

[AyKay47]

$c = eval("$b ".$a);

 

would make sense.

[Muddy_Funster]

OK, I'm clearly missing something:

$a = '*2';
$b = '10';
//$c = eval("$b ". $a);  = Unexpected $end
//$c = eval("$b ". $a.";");  = var_dump($c) returns null
//$c = eval("$b  $a;"); = var_dump($c) returns null
//$c = eval($b $a.";"); = Unexpected T_VARIABLE

$d= $b.$a ;
//$c = eval("$d;");  var_dump($c) = Null / var_dump($d) = string(4) "10*2" 

[PFMaBiSmAd]

Eval() executes a (valid) php statement. If you wanted to produce and execute $c =  10 * 1.2;, that's what the "string" you supply to the eval() statement would need to be after all the $a, $b variables have been replaced with their contents.

 

Edit: Specifically -

<?php
$a = '* 1.2';
$b = '10';

eval("\$c = $b $a;");

echo $c;

[Muddy_Funster]

Yeah, I thought I was doing that:

$a = '*2';
$b = '10';
$d= $b.$a.";";
$e = 10*2; 
$c = eval("$d");
echo "<pre>";
var_dump($c);
var_dump($d);
var_dump($e);
echo"</pre>";  

Returns:

NULL
string(5) "10*2;"
int(20)

 

Like I said, I have to be missing something here because $e looks exactly like the var_dump of $d to my eyes

[trq]

Take another look at PFMaBiSmAd's example.

[Muddy_Funster]

Excelent! So I was just being completly stupid

Note to self : assign the variable within the eval, don't assign the eval to the variable!

[xyph]

An alternate way to do this... keep in mind using EVAL can be dangerous. If you're going to allow user input, be sure to sanitize it AS STRICTLY AS POSSIBLE!

 

<?php

$a = 10;
$b = '*2';

$c = eval('return '.$a.$b.';');

echo $c;

?>

 

Check out the manual entry for eval. Under the 'Return Values' heading, it explains how eval() will populate any variables assigned by it.