how to set and update a database filed to random numbers

hello i have a table where data is stored, this table consist of a field called tnx_ref, i am trying to automatically update this field with random numbers anytime i update my table this is what i tried,

$tnx_ref = $_SESSION['genref'];




$sql = "UPDATE transactions ".
      "SET tnx_ref = '62323232'".
    "WHERE id = $id";

from the snippet above i have updated the value of that particular field "tnx_ref with "62323232", but now i want to make it different random numbers for every time i update.

how can i possible do this, any1 done such