합계구하고 싶습니다.

eval 함수 쓰면 되는데 입력값이 숫자와 연산자인지 유효성을 검사한 후 사용하시면 된다고 하네요. 보안에 취약하다고...

$test = "(7.2*6)-20";

function matheval($equation)
{
 $equation = preg_replace("/[^0-9+\-.*\/()%]/","",$equation);
    // fix percentage calcul when percentage value < 10
    $equation = preg_replace("/([+-])([0-9]{1})(%)/","*(1\$1.0\$2)",$equation);
    // calc percentage
    $equation = preg_replace("/([+-])([0-9]+)(%)/","*(1\$1.\$2)",$equation);
    // you could use str_replace on this next line
    // if you really, really want to fine-tune this equation
    $equation = preg_replace("/([0-9]+)(%)/",".\$1",$equation);
    if ( $equation == "" )
    {
      $return = 0;
    }
    else
    {
      eval("\$return=" . $equation . ";" );
    }
    return $return;
}

$test1 = matheval($test);
echo $test1;