class POP3
 {
  /**
   * The POP3 PHPMailer Version number.
   *
   * @var string
   */
  const VERSION = '6.0.7';

  /**
   * Default POP3 port number.
   *
   * @var int
   */
  const DEFAULT_PORT = 110;

  /**
   * Default timeout in seconds.
   *
   * @var int
   */
  const DEFAULT_TIMEOUT = 30;

  /**
   * Debug display level.
   * Options: 0 = no, 1+ = yes.
   *
   * @var int
   */
  public $do_debug = 0;

  /**
   * POP3 mail server hostname.
   *
   * @var string
   */
  public $host;

  /**
   * POP3 port number.
   *
   * @var int
   */
  public $port;

  /**
   * POP3 Timeout Value in seconds.
   *
   * @var int
   */
  public $tval;

  /**
   * POP3 username.
   *
   * @var string
   */
  public $username;

  /**
   * POP3 password.
   *
   * @var string
   */
  public $password;

  /**
   * Resource handle for the POP3 connection socket.
   *
   * @var resource
   */
  protected $pop_conn;

  /**
   * Are we connected?
   *
   * @var bool
   */
  protected $connected = false;

  /**
   * Error container.
   *
   * @var array
   */
  protected $errors = [];

  /**
   * Line break constant.
   */
  const LE = "\r\n";

  /**
   * Simple static wrapper for all-in-one POP before SMTP.
   *
   * @param string   $host        The hostname to connect to
   * @param int|bool $port        The port number to connect to
   * @param int|bool $timeout     The timeout value
   * @param string   $username
   * @param string   $password
   * @param int      $debug_level
   *
   * @return bool
   */
  public static function popBeforeSmtp(
  $host, 
  $port = false, 
  $timeout = false, 
  $username = '', 
  $password = '', 
  $debug_level = 0
  ) {
    $pop = new self();

    return $pop->authorise($host, $port, $timeout, $username, $password, $debug_level);
  }

  /**
   * Authenticate with a POP3 server.
   * A connect, login, disconnect sequence
   * appropriate for POP-before SMTP authorisation.
   *
   * @param string   $host        The hostname to connect to
   * @param int|bool $port        The port number to connect to
   * @param int|bool $timeout     The timeout value
   * @param string   $username
   * @param string   $password
   * @param int      $debug_level
   *
   * @return bool
   */
  public function authorise($host, $port = false, $timeout = false, $username = '', $password = '', $debug_level = 0) 
   {
    $this->host = $host;
    // If no port value provided, use default
    if (false === $port) {
      $this->port = static::DEFAULT_PORT;
    }
    else {
      $this->port = (int) $port;
    }
    // If no timeout value provided, use default
    if (false === $timeout) {
      $this->tval = static::DEFAULT_TIMEOUT;
    }
    else {
      $this->tval = (int) $timeout;
    }
    $this->do_debug = $debug_level;
    $this->username = $username;
    $this->password = $password;
    //  Reset the error log
    $this->errors = [];
    //  connect
    $result = $this->connect($this->host, $this->port, $this->tval);
    if ($result) {
      $login_result = $this->login($this->username, $this->password);
      if ($login_result) {
        $this->disconnect();

        return true;
      }
    }
    // We need to disconnect regardless of whether the login succeeded
    $this->disconnect();

    return false;
  }

  /**
   * Connect to a POP3 server.
   *
   * @param string   $host
   * @param int|bool $port
   * @param int      $tval
   *
   * @return bool
   */
  public function connect($host, $port = false, $tval = 30) 
   {
    //  Are we already connected?
    if ($this->connected) {
      return true;
    }

    //On Windows this will raise a PHP Warning error if the hostname doesn't exist.
    //Rather than suppress it with @fsockopen, capture it cleanly instead
    set_error_handler([$this, 'catchWarning']);

    if (false === $port) {
      $port = static::DEFAULT_PORT;
    }

    //  connect to the POP3 server
    $this->pop_conn = fsockopen(
    $host, //  POP3 Host
    $port, //  Port #
    $errno, //  Error Number
    $errstr, //  Error Message
    $tval
    ); //  Timeout (seconds)
    //  Restore the error handler
    restore_error_handler();

    //  Did we connect?
    if (false === $this->pop_conn) {
      //  It would appear not...
      $this->setError(
      "Failed to connect to server $host on port $port. errno: $errno; errstr: $errstr"
      );

      return false;
    }

    //  Increase the stream time-out
    stream_set_timeout($this->pop_conn, $tval, 0);

    //  Get the POP3 server response
    $pop3_response = $this->getResponse();
    //  Check for the +OK
    if ($this->checkResponse($pop3_response)) {
      //  The connection is established and the POP3 server is talking
      $this->connected = true;

      return true;
    }

    return false;
  }

  /**
   * Log in to the POP3 server.
   * Does not support APOP (RFC 2828, 4949).
   *
   * @param string $username
   * @param string $password
   *
   * @return bool
   */
  public function login($username = '', $password = '') 
   {
    if (!$this->connected) {
      $this->setError('Not connected to POP3 server');
    }
    if (empty($username)) {
      $username = $this->username;
    }
    if (empty($password)) {
      $password = $this->password;
    }

    // Send the Username
    $this->sendString("USER $username" . static::LE);
    $pop3_response = $this->getResponse();
    if ($this->checkResponse($pop3_response)) {
      // Send the Password
      $this->sendString("PASS $password" . static::LE);
      $pop3_response = $this->getResponse();
      if ($this->checkResponse($pop3_response)) {
        return true;
      }
    }

    return false;
  }

  /**
   * Disconnect from the POP3 server.
   */
  public function disconnect() 
   {
    $this->sendString('QUIT');
    //The QUIT command may cause the daemon to exit, which will kill our connection
    //So ignore errors here
    try {
      @fclose($this->pop_conn);
    }
    catch (Exception $e) {
      //Do nothing
    }
  }

  /**
   * Get a response from the POP3 server.
   *
   * @param int $size The maximum number of bytes to retrieve
   *
   * @return string
   */
  protected function getResponse($size = 128) 
   {
    $response = fgets($this->pop_conn, $size);
    if ($this->do_debug >= 1) {
      echo 'Server -> Client: ', $response;
    }

    return $response;
  }

  /**
   * Send raw data to the POP3 server.
   *
   * @param string $string
   *
   * @return int
   */
  protected function sendString($string) 
   {
    if ($this->pop_conn) {
      if ($this->do_debug >= 2) { //Show client messages when debug >= 2
        echo 'Client -> Server: ', $string;
      }

      return fwrite($this->pop_conn, $string, strlen($string));
    }

    return 0;
  }

  /**
   * Checks the POP3 server response.
   * Looks for for +OK or -ERR.
   *
   * @param string $string
   *
   * @return bool
   */
  protected function checkResponse($string) 
   {
    if (substr($string, 0, 3) !== '+OK') {
      $this->setError("Server reported an error: $string");

      return false;
    }

    return true;
  }

  /**
   * Add an error to the internal error store.
   * Also display debug output if it's enabled.
   *
   * @param string $error
   */
  protected function setError($error) 
   {
    $this->errors [] = $error;
    if ($this->do_debug >= 1) {
      echo '<pre>';
      foreach ($this->errors as $e) {
        print_r($e);
      }
      echo '</pre>';
    }
  }

  /**
   * Get an array of error messages, if any.
   *
   * @return array
   */
  public function getErrors() 
   {
    return $this->errors;
  }

  /**
   * POP3 connection error handler.
   *
   * @param int    $errno
   * @param string $errstr
   * @param string $errfile
   * @param int    $errline
   */
  protected function catchWarning($errno, $errstr, $errfile, $errline) 
   {
    $this->setError(
    'Connecting to the POP3 server raised a PHP warning:' .
      "errno: $errno errstr: $errstr; errfile: $errfile; errline: $errline"
      );
  }
}