crating unique numbers

<?php
function lm_unique_number($limitnum){
return substr(number_format(time() * rand(),0,'',''),0,$limitnum);
}
$unique=new connect();
$uniquenumber=$unique->conn->prepare("SELECT cmi_id FROM lm_college_meta_info ");
$uniquenumber->execute();
while($uniquenumberarray=$uniquenumber->fetch()){
$i_lm_unique=lm_unique_number(8);
$unique->conn->exec("UPDATE lm_college_meta_info SET i_lm_unique =$i_lm_unique WHERE cmi_id='$uniquenumberarray[0]' ");
}
?>