Store plus and minus in a variable PHP -

how can solve problem:

if ($variable == 1) {     $math = "-"; } else {     $math = "+"; }  $numberone = 10; $numbertwo = 10;  $result = $numberone $math $numbertwo; 

this doesn´t work, there way solve this?

this work example. subtraction same adding negative. far safer alternative of using eval.

if ($variable == 1) {     $modifier = -1; } else {     $modifier = 1; }  $numberone = 10; $numbertwo = 10;  $result = $numberone + ($numbertwo * $modifier);