This script is designed to create .eml file that will be opened with any default email client software like thunderbird or microsoft outlook.

Script

$email='From: FROM_EMAIL_ADDRESS
MIME-Version: 1.0
To: TO_EMAIL_ADDRESS
CC: CC_EMAIL_ADDRESS
BCC: BCC_EMAIL_ADDRESS
Subject: EMAIL_SUBJECT
Content-Type: multipart/mixed; boundary="080107000800000609090108"

This is a message with multiple parts in MIME format.
--080107000800000609090108
Content-Type: text/html

EMAIL_BODY
--080107000800000609090108
Content-Type: application/octet-stream;name="EMAIL_ATTACH_FILENAME"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;filename="EMAIL_ATTACH_FILENAME"';

$handle=fopen('email.eml','w');
fwrite($handle, $email);
fclose($handle);

Output

email.eml