How to send email with attachment using PHP?

I'd suggest to use a library for sending eMails as it will handle all the header related stuff. Have a look at Zend_Mail. Sending attachments is as easy as

$mail = new Zend_Mail();

$at = $mail->createAttachment($myImage);
$at->type        = 'image/gif';
$at->disposition = Zend_Mime::DISPOSITION_INLINE;
$at->encoding    = Zend_Mime::ENCODING_8BIT;
$at->filename    = 'test.gif';

$mail->send();

You need to put all of your multipart message into $email_message:

$semi_rand = md5(time());
$mime_boundary = "==Multipart_Boundary_x{$semi_rand}x";

$headers = "From: ".$email_from;
$headers .= "\nMIME-Version: 1.0\n" .
"Content-Type: multipart/mixed;\n" .
" boundary=\"{$mime_boundary}\"";

$email_message .= "This is a multi-part message in MIME format.\n\n" .
"--{$mime_boundary}\n" .
"Content-Type:text/html; charset=\"iso-8859-1\"\n" .
"Content-Transfer-Encoding: 7bit\n\n" .
"--{$mime_boundary}\n" .
"Content-Type: {$fileatt_type};\n" .
" name=\"{$fileatt_name}\"\n" .
"Content-Transfer-Encoding: base64\n\n" .
chunk_split(base64_encode($data)) .
"--{$mime_boundary}--\n";