pre-populated dropdown menu, multiple options from Mysql database (php)

pre-populated dropdown menu, multiple options from Mysql database (php)

Newbie here... I tried to adapt my code from here
The list is populated properly but I can't get it to pre-select. What am I
doing wrong? Thanks in advance!
$q = "SELECT cat_id FROM category_user WHERE cat_id=$cat_id";
while ($row = mysqli_fetch_array($q)) {
$cat = (int)$row['cat_id'];
}
$q = "SELECT cat_id, cat FROM category";
$r = mysqli_query ($dbc, $q);
if (mysqli_num_rows($r)> 0) {
while ($row = mysqli_fetch_array ($r, MYSQLI_NUM)) {
echo "<option value=\"$row[0]\"";
$selected = '';
$cid=(int)$row[0];
if ($cid=$cat) {
$selected='selected="selected"';
echo $selected;
echo ">$row[1]</option>\n";
}else{
//Check for stickyness
if (isset($_POST['category'])&&($_POST['category']==
$row[0]))
echo 'selected="selected"';
echo ">$row[1]</option>\n";
}
}
}