Mysql - Select WHERE AND - VARIABLE does not work

I have the following problem:

//$sql = "SELECT name, number, letter FROM queues WHERE number='100' AND letter='S'";
      
$sql = "SELECT name, number, letter FROM queues WHERE number='".$contentnumber."' AND  
      letter='".$contentletter."'";

See both queries above.

Both give the following out when I execute: var_dump($sql);

string(121) "SELECT name, number, letter FROM queues WHERE number='100' AND letter='S'" 

Output by both is the same but with the version of the variables data from the database is not loaded. But with the version where I put the values for numbers and letters, it works fine.

Does anyone know where the problem or the mistake is?

Here is a little bit detailed:

ob_start();
echo "{{tokens[0].token_letter}}";
$contentletter = ob_get_clean();

ob_start();
echo "{{tokens[0].token_number}}";
$contentnumber = ob_get_clean();
$conn = mysqli_connect($serverName, $userName, $password, $dbName);
if (!$conn) { die("Connection failed: " . mysqli_connect_error()); }
$sql = "SELECT name, number, letter FROM queues WHERE
number='".$contentnumber."' AND letter='".$contentletter."'";
var_dump($sql);