php 创建指定前缀的字符串

<?php 
header("content-type=text/html;charset=utf-8");
function create_guid($pre_num){
    $charid = strtoupper(md5(uniqid(mt_rand(), true)));
    $hyphen = chr(45); //"-"连接符
    $d=$pre_num.$hyphen;//前缀-
   $uuid="" 
    .substr($charid,0,12);
$uuid=$d.$uuid;
    return $uuid;
}//endfunction

echo create_guid(wx);
?>