Mysqli SUM query does not return result

Question

I am trying to sum up a column using the mysqli SUM method. The problem is that my query does not return results. My code is this:

   $sql = 'SELECT SUM(size) as soma, porta FROM `'
        .$this->options['db_table'].'` WHERE `porta`=?';
    $query = $this->db->prepare($sql);
    $query->bind_param('s', $file->porta);
    $query->execute();
    $query->bind_result(
        $soma
    ); 

   $query->close();

   echo $soma;

What could be wrong? Remembering that all other queries without using SUM() work perfectly.

Answer 1

Lacked use fetch:

$sql   = 'SELECT SUM(size) as soma, porta FROM `'.$this->options['db_table'].'` WHERE `porta`=?';
$query = $db->prepare($sql);
$query->bind_param('s', $file->porta);
$query->execute();
$query->bind_result($soma); 

if($query->fetch()){
    echo 'result is ' . $soma;
}
$query->close();