Filtering MySQL query. "AND" operator

Filtering MySQL query. "AND" operator

I'm getting excessive rows selected from the db. However, when i try to
filter the data with AND lt_num it does not select anything at all. What
could be the problem?
$w = $_POST['pal_num'];
$w = "'".implode("','",$_POST['pal_num'])."'";
$r = mysql_query("SELECT * FROM pl_tab WHERE pal_num AND lt_num in
(".$weights.");");