Is MD5 a secure cryptographic hash function?

MD5 hashes are no longer considered cryptographically secure methods and should not be used for cryptographic authentication, according to IETF. I have been working with MD5 and Sha512, to answer the question - Is MD5 secure, I had put my little hacker expert on it, until it both hit us like a thunder! The most simple and effective way to stop "Mass Attacks'' like the ones described above, is setting a limiter of login attempts, meaning :

function checkbrute($user_id, $mysqli) {
 // Get timestamp of current time
   $now = time();
   // All login attempts are counted from the past 2 hours. 
   $valid_attempts = $now - (2 * 60 * 60); 
   if ($stmt = $mysqli->prepare("SELECT time FROM login_attempts WHERE user_id = ? AND time > '$valid_attempts'")) { 
  $stmt->bind_param('i', $user_id); 
  // Execute the prepared query.
  $stmt->execute();
  $stmt->store_result();
  // If there has been more than 5 failed logins
  if($stmt->num_rows > 5) {
     return true;
  } else {
     return false;
  }
}
}

The maximum number of combinations the hacker wants to try, he is limited to 5 fails per 2 hours. You can even block the user after X attempts.