Generate a million unique random 12 digit numbers

I need to generate close to a million(100 batches of 10000 numbers) unique and random 12 digit codes for a scratch card application. This process will be repeated and will need an equal number of codes to be generated everytime.

Also the generated codes need to be entered in a db so that they can be verified later when a consumer enters this on my website. I am using PHP and Mysql to do this. These are the steps I am following

a) Get admin input on the number of batches and the codes per batch
b) Using for loop generate the code using mt_rand(100000000000,999999999999)
c) Check every time a number is generated to see if a duplicate exists in the db and if not add to results variable else regenerate.
d) Save generated number in db if unique
e) Repeat b,c, and d over required number of codes
f) Output codes to admin in a csv

Code used(removed most of the comments to make it less verbose and because I have already explained the steps earlier):

$totalLabels = $numBatch*$numLabelsPerBatch;
// file name for download
$fileName = $customerName."_scratchcodes_" . date('Ymdhs') . ".csv";
$flag = false;
$generatedCodeInfo = array();
// headers for download
header("Content-Disposition: attachment; filename=\"$fileName\"");
header("Content-Type: application/vnd.ms-excel");
$codeObject = new Codes();
//get new batch number 
$batchNumber = $codeObject->getLastBatchNumber() + 1;
$random = array();
for ($i = 0; $i < $totalLabels; $i++) {
    do{
        $random[$i] = mt_rand(100000000000,999999999999); //need to optimize this to reduce collisions given the databse will be grow
    }while(isCodeNotUnique($random[$i],$db));
    $codeObject = new Codes();
    $codeObject->UID = $random[$i];
    $codeObject->customerName = $customerName;
    $codeObject->batchNumber = $batchNumber;
    $generatedCodeInfo[$i] = $codeObject->addCode();

    //change batch number for next batch
    if($i == ($numLabelsPerBatch-1)){$batchNumber++;}


    //$generatedCodeInfo[i] = array("UID" => 10001,"OID"=>$random[$i]);
    if(!$flag) {
        // display column names as first row
        echo implode("\t", array_keys($generatedCodeInfo[$i])) . "\n";
        $flag = true;
    }
    // filter data
    array_walk($generatedCodeInfo[$i], 'filterData');
    echo implode("\t", array_values($generatedCodeInfo[$i])) . "\n";


}


function filterData(&$str)
{
    $str = preg_replace("/\t/", "\\t", $str);
    $str = preg_replace("/\r?\n/", "\\n", $str);
    if(strstr($str, '"')) $str = '"' . str_replace('"', '""', $str) . '"';
}

function isCodeNotUnique($random){
    $codeObject = new Codes();
    $codeObject->UID = $random;
    if(!empty($codeObject->getCodeByUID())){
        return true;
    }
    return false;
}

Now this is taking really long to execute and I believe is not optimal.

1) How can I optimize so that the unique random numbers are generated quickly?
2) Will it be faster if the numbers were instead generated in mysql or other way rather than php and if so how do I do that?
3) When the db starts growing the duplicate check in step b will be really time consuming so how do I avoid that?
4) Is there a limit on the number of rows in mysql?