How to check if random created number already exist

I am creating a unique reference number using php.

I would like to check if the number already exist in the database.

Here is my current code:

<?php

function generateRandomString($length = 3) {
$characters = 'ABCDEFGHJKLMNOPQRSTVWXYZ';//No "I"
$charactersLength = strlen($characters);
$randomString = '';
for ($i = 0; $i < $length; $i++) {
    $randomString .= $characters[rand(0, $charactersLength - 1)];
}
return $randomString;
}

$var = generateRandomString();


$random = rand(1000, 9999);
$randoms = rand(1000, 9999);

$tokennr = "I$var-$random" . "0" . $randoms;


require_once('connect_pdo.php');
header('Content-Type: text/html; charset=utf-8');
$stmt = $conn->prepare("SELECT UniqueNumber FROM `MyTable` ");
        $stmt->execute();
while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
   $uniqueref = $row["UniqueRefNr"];
}
     if($tokennr == $uniqueref){
         $tokennrs = "I$var-14$random" . "6" . $randoms;
         $token = $tokennrs;
     }
else{
$token = $tokennr;
}
echo $token;

?>

It seems that the code still uses the unique number even if it already exist. (I hard-coded some test numbers).

I do not want to use a number if it already exist.