$file_name    = "$datei_name.pdf";
$file         = "./rechnungen/$file_name";
$from         = "mail@novara-it.de";
$to           = "mail@novara-it.de";
$message      = "Mitteilung";
$betreff      = "Ihre angeforderte Rechnung";
$boundary     = strtoupper(md5(uniqid(time())));
$mail_header  = "From:novara-it <billing@novara-it.de>\n";
$mail_header .= "MIME-Version: 1.0";
$mail_header .= "\nContent-Type: multipart/mixed; boundary=$boundary";
$mail_header .= "\n\nThis is a multi-part message in MIME format "; 
$mail_header .= "--  Dies ist eine mehrteilige Nachricht im MIME-Format";
$mail_header .= "\n--$boundary"; 
$mail_header .= "\nContent-Type: text/plain";
$mail_header .= "\nContent-Transfer-Encoding: 8bit";
$mail_header .= "\n\n$message";
$file_content = fread(fopen($file,"r"),filesize($file));
$file_content = chunk_split(base64_encode($file_content));
$mail_header .= "\n--$boundary";
$mail_header .= "\nContent-Type: application/octetstream; name=\"$file_name\"";
$mail_header .= "\nContent-Transfer-Encoding: base64";
$mail_header .= "\nContent-Disposition: attachment; filename=\"$file_name\"";
$mail_header .= "\n\n$file_content";
$mail_header .= "\n--$boundary--";

mail($to,$betreff,$message,$mail_header);