PHP - How to create a secure 10 digits key

Well, i need a way to create a random unique key of 10 digits that matchs to the follow pattern:

LLNNNLLNNN

where L is a letter and N is a number.

I have found a solution but i'm looking for some smart opinion. My solution was (php):

        $alhpabet = ['A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','X','W','Y','Z']; //26 letras         
        $transaction_id = ''; //string

        //Two first letters
        $transaction_id .= $alhpabet[secure_random_number(0,25)];
        $transaction_id .= $alhpabet[secure_random_number(0, 26)];
        //3 first numbers (3 digits)
        $transaction_id .= str_pad(secure_random_number(0, 999),3,'0',STR_PAD_LEFT);
        //last letters
        $transaction_id .= $alhpabet[secure_random_number(0,25)];
        $transaction_id .= $alhpabet[secure_random_number(0, 26)];
        //last numbers
        $transaction_id .= str_pad(secure_random_number(0, 999),3,'0',STR_PAD_LEFT);

My function to generate secure_random_number is:

function secure_random_number($min, $max){
$range = $max - $min;
if($range == 0)
    return $min; // not so random...
$log = log($range, 2);
$bytes = (int) ($log / 8) + 1; // length in bytes
$bits = (int) $log + 1; // length in bits
$filter = (int) (1 << $bits) - 1; // set all lower bits to 1
do{
    $rnd = hexdec(bin2hex(openssl_random_pseudo_bytes($bytes, $s)));
    $rnd = $rnd & $filter; // discard irrelevant bits
}while($rnd >= $range);
return $min + $rnd;
}

ps: i get this somewhere in here, but forget to note the credits...

NOTE: I'm not worried about performance.