PHP Pear Mail Returns Fatal Error: Failed opening required 'Mail.php'

How to include mail.php for using PHP Pear Mail. I'm using the following code in test.php file:

    require_once "Mail.php";

    $from = "<test@gmail.com>";
    $to = "<testing@gmail.com>";
    $subject = "Hi!";
    $body = "Hi,\n\nHow are you?";

    $host = "ssl://smtp.gmail.com";
    $port = "465";
    $username = "<testtest@gmail.com>";
    $password = "testtest";

    $headers = array ('From' => $from,
      'To' => $to,
      'Subject' => $subject);
    $smtp = Mail::factory('smtp',
      array ('host' => $host,
        'port' => $port,
        'auth' => true,
        'username' => $username,
        'password' => $password));

    $mail = $smtp->send($to, $headers, $body);

    if (PEAR::isError($mail)) {
      echo("<p>" . $mail->getMessage() . "</p>");
    } else {
      echo("<p>Message successfully sent!</p>");
    }

and the following error is encountered via this code:

  Warning: require_once(Mail.php) [function.require-once]: failed to open stream: No such file or directory in D:\Hosting\6525150\html\test.php on line 3

  Fatal error: require_once() [function.require]: Failed opening required 'Mail.php' (include_path='.;C:\php5\pear') in D:\Hosting\6525150\html\test.php on line 3

Can someone tell me what is the problem?

Check if pear is installed in the system . If yes then specify path to Pear installation directory in php.ini include_path directive

You need to install PEAR and PEAR MAIL packages to get it working