php字符串算术表达式计算

$aa = "{1}*{2}-{3}";
$farr = array('/{1}/','/{2}/','/{3}/');
$tarr = array(3,4,10);
$str = preg_replace( $farr,$tarr,$aa);
echo $str;        //结果：3*4-10
echo eval('return '.$str.';');   //结果:2

来源：http://blog.51yip.com/php/1846.html