<?php
if(isset($_GET['add_favorite'])) {
if(isset($_COKKIE['fav_ads_ids'])){
	$ads = implode(',', json_decode($_COOKIE['fav_ads_ids']));
	$favorite_id = SELECT id FROM suaTabela WHERE id NOT IN ({$ads}) ORDER BY RAND() LIMIT 1
	$arr = json_decode($_COOKIE['fav_ads_ids']);
}else{
	$favorite_id = SELECT id FROM suaTabela ORDER BY RAND() LIMIT 1
	$arr = [];
}
 
$arr[] = $favorite_id['id'];
$arr = json_encode($arr);
setcookie("fav_ads_ids", $arr, time() + (86400 * 30), "/");
}
print_r($_COOKIE['fav_ads_ids']);

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