par que sirve

[blas9]

/*******************Este es todo el codigo que encontre para enviar un correo con unn archivo adjunto pero no me manda nada y no se porque si alguien me puede explicar popr que se les agradeceria mucho *************/
$to=$_POST["Correo_Electronico"];
$subject=$_POST['asunto'];
$from = stripslashes($_POST['fromname'])."<".stripslashes($_POST['fromemail']).">";
$mime_boundary="==Multipart_Boundary_x".md5(mt_ran d())."x";
$tmp_name = $_FILES['filename']['tmp_name'];
$type = $_FILES['filename']['type'];
$name = $_FILES['filename']['name'];
$size = $_FILES['filename']['size'];
$message = $_POST['mensaje'];
if (file_exists($tmp_name)){
if(is_uploaded_file($tmp_name)){
$file = fopen($tmp_name,'rb');
$data = fread($file,filesize($tmp_name));
fclose($file);
$data = chunk_split(base64_encode($data));
}
$headers = "From: $from\r\n" .
"Content-Type: multipart/mixed;\r\n" .
" boundary=\"{$mime_boundary}\"";
$message = "This is a multi-part message in MIME format.\n\n" .
"--{$mime_boundary}\n" .
"Content-Type: text/plain; charset=\"iso-8859-1\"\n" .
"Content-Transfer-Encoding: 7bit\n\n" .
$message . "\n\n";
$message .= "--{$mime_boundary}\n" .
"Content-Type: {$type};\n" .
" name=\"{$name}\"\n" .
"Content-Transfer-Encoding: base64\n\n" .
$data . "\n\n" .
"--{$mime_boundary}--\n";
if (@mail($to, $subject, $message, $headers)) {

echo "Correo enviado<br>";
} else{
echo "Fallo en el envío";}
}