HTML
<select multiple name="location[]" size="2">
PHP
$w = array();
$where = '';
foreach ($_POST['location'] as $loc){
$loc = mysql_real_escape_string($loc);
$w[] = "location = '$loc'";
}
if (!empty($_POST['type'])){
$w[] = "type LIKE '%".mysql_real_escape_string($_POST['type'])."%'";
}
if ($w) $where = implode(' OR ',$w);