$kategoriq = $_GET["cat"]; или $kategoriq = $_POST["cat"];
$kategoriq = 0+$kategoriq; или $kategoriq = (int)$kategoriq;
$res = mysql_query("SELECT * FROM ПОТРЕБИТЕЛИ WHERE categoriq='".mysql_real_escape_string($kategoriq)."' ORDER BY id);
while($row = mysql_fetch_array($res)) {
echo $row[id] . " " . $row[username] . "<br />";
}