PHP - suma con php y mysql

$query = "SELECT nombre FROM tabla where alta='si'  " ;

  $result = mysql_query($query);

$res = mysql_query("SELECT SUM(horas) as total  FROM tabla ");

$total = mysql_result($res,0);

echo "<table border>";

    while ($a = mysql_fetch_array($result)){

echo "<tr>";

echo "<td> ".$a['nom']."</td><td>$total</td>";

echo "</tr>";

}

echo '</table>';

<?php

$db = mysqli_connect("localhost", "USER", "PASSWORD", "DATABASE");

/* verificar la conexión */

if (mysqli_connect_errno()) {

    die("Conexión fallida:" . mysqli_connect_error());

}

$sql = "SELECT SUM(horas) as total, nombre  FROM tabla WHERE alta='si' GROUP BY(nombre)";

$result = mysqli_query($db, $sql);

echo "<table border>";

while ($a = mysqli_fetch_assoc($result)){

echo "<tr>";

echo "<td>".$a['nombre']."</td>";

echo "<td>".$a['total']."</td>";

echo "</tr>";

}

echo '</table>';

WHERE alta='si'