Undefined variable when declaring with concatenation

I get an undefined variable $ar, $pr and $af.

$sql = mysqli_query($connection, "Select empno, username, password, access_level from personaltab where access_level='ADMIN'");

$cnt = mysqli_num_rows($sql);
$i=0;

while ($r=mysqli_fetch_array($sql)){
$md = md5($r['username']."!@#$%^&*()_+|");
$ar .= $md.", ";
$mdp = md5($r['password']."|+_)(*&^%$#@!<>?:{}[]=-");
$pr .= $mdp.", ";
$af .= $r['empno'].", ";
}