mssql-php查询mssql表行数，为什么没输出？？

$ms_host="localhost";
$ms_user="sa";
$ms_pass="sa";
$ms_data="his";
$ms_conn=mssql_connect($ms_host,$ms_user,$ms_pass);
mssql_select_db($ms_data,$ms_conn);

$sql ="select * from zsb_customer ";

echo mssql_num_rows(mssql_query($sql));