怎么获取sql的SUM()值？紧急本人太菜

如何获取sql的SUM()值？紧急！在线等！本人太菜
有一个表test1
字段名num
有三个字段 111 222 333

$sql=mysql_query("select SUM(num) from test1");

$a=xxx($sql);


接下来该用哪个函数xxx把666这个数取出来赋予变量啊？？？

万分感激

------解决方案--------------------
sql=mysql_query("select SUM(num) as t_sum from test1");
$res = mysql_fetch_array()
echo $res[t_sum]
------解决方案--------------------
$a=mysql_fetch_array($sql);
echo "

";<br>print_r($a);<br>echo "

";
------解决方案--------------------
sql=mysql_query("select SUM(num) as sum from test1"); 
$res = mysql_fetch_array() 
echo $res[sum]