Overcoming PHP Memory usage issues for array functions - php

Question

I have this PHP code below to generate a set of 12 digit unique random numbers(ranging from 100000 to a million) and save it in db. I am first fetching the existing codes from MySQL db(right now there are already a million of them), flipping the array, generating a new codes. Later I use array_diff_key and array_keys on $random and $existingRandom to get the new codes which are to be saved back to db.
// total labels is the number of new codes to generate
//$totalLabels is fetched as user input and could range from 100000 to a million
$codeObject = new Codes();
//fetch existing codes from db
$codesfromDB = $codeObject->getAllCodes();
$existingRandom = $random = array();
$existingRandom = $random = array_flip($codesfromDB);
$existingCount = count($random); //The codes you already have
do {
$random[mt_rand(100000000000,999999999999)] = 1;
} while ((count($random)-$existingCount) < $totalLabels);
$newCodes = array_diff_key($random,$existingRandom);
$newCodes = array_keys($newCodes);
The issue I am facing is that the array_flip function is running out of memory and causing my program to crash Error
"Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 72 bytes)"
My questions are below:
1) Can someone help me understand why the array_flip is running out of memory. Memory limit in php.ini file is 256M. Please show me calculation of the memory used by the function if possible. (Also if array_flip passes array_diff_key and array_keys run out of memory)
2) How do I optimize the code so that the memory used is under the limit. I even tried to break the array_flip operation in smaller chunks but even that is running out of memory.
$size = 5000;
$array_chunk = array_chunk($codesfromDB, $size);
foreach($array_chunk as $values){
$existingRandom[] = $random[] = array_flip($values);
}
3) Is what I am doing optimal would it be fair to further increase the memory limit in php.ini file. What are the things to keep in mind while doing that.
Here is my query as well to fetch the existing codes from db if needed:
$sql = "SELECT codes FROM code";
$stmt = $this->db->prepare($sql);
$stmt->execute();
$result = $stmt->fetchAll(PDO::FETCH_COLUMN, 0);
return $result;