PHP code for generating decent-looking coupon codes (mix of letters and numbers)

$chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
$res = "";
for ($i = 0; $i < 10; $i++) {
    $res .= $chars[mt_rand(0, strlen($chars)-1)];
}

substr(base_convert(sha1(uniqid(mt_rand())), 16, 36), 0, 10)

<?php
    echo strtoupper(uniqid());
?>

function randr($j = 8){
$string = "";
    for($i=0;$i < $j;$i++){
        srand((double)microtime()*1234567);
        $x = mt_rand(0,2);
        switch($x){
            case 0:$string.= chr(mt_rand(97,122));break;
            case 1:$string.= chr(mt_rand(65,90));break;
            case 2:$string.= chr(mt_rand(48,57));break;
        }
    }
return strtoupper($string); //to uppercase
}

CX00019 QZ0001C
CX0001A QZ0001D
CX0001B QZ0001E

Promo60
NoSetupFee
YELLOWGORILLA82

$size = 12;

$string = strtoupper(substr(md5(time().rand(10000,99999)), 0, $size));

function generateCouponCode($length = 8) {
  $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
  $ret = '';
  for($i = 0; $i < $length; ++$i) {
    $random = str_shuffle($chars);
    $ret .= $random[0];
  }
  return $ret;
}

   <?php
//To Pull 8 Unique Random Values Out Of AlphaNumeric

//removed number 0, capital o, number 1 and small L
//Total: keys = 32, elements = 33
$characters = array(
"A","B","C","D","E","F","G","H","J","K","L","M",
"N","P","Q","R","S","T","U","V","W","X","Y","Z",
"1","2","3","4","5","6","7","8","9");

//make an "empty container" or array for our keys
$keys = array();

//first count of $keys is empty so "1", remaining count is 1-7 = total 8 times
while(count($keys) < 8) {
    //"0" because we use this to FIND ARRAY KEYS which has a 0 value
    //"-1" because were only concerned of number of keys which is 32 not 33
    //count($characters) = 33
    $x = mt_rand(0, count($characters)-1);
    if(!in_array($x, $keys)) {
       $keys[] = $x;
    }
}

foreach($keys as $key){
   $random_chars .= $characters[$key];
}
echo $random_chars;
?>

$length = 9;
$code   = (strtoupper(substr(md5(time()), 0, $length)));

 $voucher_no =  date('ymd') . rand(1000, 9999);
 while(SapItem::where('voucher_no', $voucher_no)->exists()){
  $voucher_no = date('ymd') . rand(1000, 9999);
 }