$len = 16;
$base='ABCDEFGHKLMNOPQRSTWXYZabcdefghjkmnpqrstwxyz123456789';
$max=strlen($base)-1;
$activatecode='';
mt_srand((double)microtime()*1000000);
while (strlen($activatecode)<$len+1)
  $activatecode.=$base{mt_rand(0,$max)};
 
echo $activatecode;