Reverse Polish notation PHP implementation

function rpn($expr){
    if(is_string($expr)){
        $expr = explode(" ", $expr);
    }

    $stack = [];
    foreach($expr as $e){
        if(is_numeric($e)){
            $stack[] = $e;
        } else {
            $n1 = array_pop($stack);
            $n2 = array_pop($stack);
            $result = null;
            if($e == "+"){
                $result = $n2 + $n1;
            } elseif($e == "*"){
                $result = $n2 * $n1;
            } elseif($e == "-"){
                $result = $n2 - $n1;
            } elseif($e == "/"){
                $result = $n2 / $n1;
            }
            array_push($stack, $result);
        }
    }

    if(count($stack) != 1){
        throw new TypeError("Invalid rpn expression");
    }

    return array_pop($stack);
}