[php] Debugging PHP Mail() and/or PHPMailer

I'm quite stuck with a problem sending mail from a PHP script. Some data:

• Shared hosting, no SSH access, only hosting provider panel
• PHP version 5.2.5
• Last year I built a site which had no problems sending mail with the same hosting
• Let's say the domain is “domain.com” and my private address is “[email protected]” for anonimity's sake in the following code.

Here's the code:

<?php
error_reporting(E_ALL); 
ini_set("display_errors", 1);

$to = "[email protected]";
$subject = "Hi";
$body = "Test 1\nTest 2\nTest 3";
$headers = 'From: [email protected]' . "\r\n" .
    'errors-to: [email protected]' . "\r\n" .
    'X-Mailer: PHP/' . phpversion();

if (mail($to, $subject, $body, $headers)) {
    echo("Message successfully sent");
} else {
    echo("Message sending failed");
}

require('class.phpmailer.php');
$message = "Hello world";
$mail = new PHPMailer();
$mail->CharSet = "UTF-8";
$mail->AddAddress("[email protected]", "Agos");
$mail->SetFrom("[email protected]","My Site");
$mail->Subject = "Test Message";
$mail->Body = $message;
$mail->Send();
?>

And here is what I get:

Message sending failed Could not instantiate mail function.

Which is baffling to say the least. Is there anything I can do to get at least some more meaningful errors? Why is code from the class showing up in my file?