Undefined index: count(*) PDO

So I'm recreating link generation function from mysql to PDO and I have a problem.

    $sth = $resDB->prepare("SELECT count(*) FROM downloads WHERE downloadkey = '{$strKey}' LIMIT 1");
$arrCheck = $sth->fetchAll();
if($arrCheck['count(*)']){
    //key already in use
    return createKey();
}else{
    //key is OK
    return $strKey;
}

Everything goes well until this line: if($arrCheck['count(*)'])

I get the error like this: Undefined index: count(*)

I just don't know how to change that line to PDO. Can you help me ?