Auto Number PHP MySQL

<?php
function get_new_id($fieldname,$table,$kodeawal){
 $q = "Select ".$fieldname." from ".$prefix.$table." ORDER BY ".$fieldname." DESC LIMIT 1";
 $res=mysql_query($q) or die(mysql_error());
 $row = mysql_fetch_array($res);
 $cekQ=$row[$fieldname];
 $awalQ=substr($cekQ,0-8);
 $next=$awalQ+1;
 $jid=strlen($next);
 if($jid==1)
 { $no='0000000'; }
    elseif($jid==2)
    { $no='000000'; }
    elseif($jid==3)
    { $no='00000'; }
    elseif($jid=4)
    { $no='0000'; }
 $br=$kodeawal.$no.$next;
 return $br;
}
?>

//HOW TO WORK
<?php echo get_new_id("id","barang","BR"); ?>