user_id (primary, AI)
username (unique)
email (unique)
password
field 1
field 2
...

LOCK TABLES users WRITE;
// users provided username and email are already taken?
SELECT COUNT(username) username, (SELECT COUNT(email) FROM users WHERE email = 'batman@heroes.com') email FROM users WHERE username = 'batman'

$res = fetch_array;
if($res['username'] == 0 && $res['email'] == 0){
   INSERT INTO users (username,email,password) VALUES ('batman','batman@heroes.com','imtheonlytruehero')

   if(affected_rows == 1)
      $username = null;
      $email= null;
      $registration = true;
   }
   else {
      $username = null;
      $email= null;
      $registration = false;
   }
}
else {
   $username = ($res['username'] > 0) ? false : true;
   $email = ($res['email'] > 0) ? false : true;
   $registration = false;
}
UNLOCK TABLES;

START TRANSACTION;
INSERT INTO users (username,email,password) VALUES ('batman','0','imtheonlytruehero');

if(affected_rows > 0) {
   $username = true;
   UPDATE users SET email='batman@heroes.com' WHERE username='batman';
   if(affected_rows > 0) {
      $email = true;
      $registration = true;
      COMMIT;
   }
   else {
      $email = false;
      $registration = true;
      ROLLBACK;
   }
}
else {
   $username = false;
   $email = null;
   $registration = false;
   ROLLBACK;
}

while (1) {
    START TRANSACTION;
    SELECT * FROM users WHERE username = ? OR email = ?;

    if (something_found) {
        // figure out what was found — email, login or both
        // ...
    } else {
        try {
            INSERT INTO users …;
        } catch (UniqueViolation) {
            // somebody else could have inserted a record with the same
            // username/email after our SELECT query
            ROLLBACK;
            // try again
            continue;
        }
    }
    COMMIT;
    break;
}

$result =  $db->query('SELECT COUNT(id) as total FROM USER WHERE username = "batman" or email = "batman@heroes.com"');
$row = $result->fetch_assoc();
if($row['total'] > 0}{
  // redirect back to form, exit here
}

try{
  // no user with email, username exists, so try to insert that user
  $db->query('INSERT INTO USER ...');
  $registration = true;
}
catch(Exception e){
   // If a different user with same username or email registered in
   // the mean time then redirect to form.
}