Code:

      $headers = "From: $from\n"
       ."Reply-To: $from\n";
    $headers  .= "MIME-Version: 1.0\n";
    $headers .= "Content-Type: multipart/mixed; boundary=\"MIME_BOUNDRY\"\n";
    $message = "--MIME_BOUNDRY\n";
    $message .= "Content-type: text/html; charset=utf-8\n";
    $message .= "\n";
    $message .= $body;
    $message .= "\n";
    $filename =  $attachment['filename'];
    $name =  $attachment['name'];
    
    $file_url = ITEM_FILE_PATH . $filename;
    $fp = fopen($file_url,"rb");
    $attachment = fread($fp, filesize($file_url));
    $attachment = chunk_split(base64_encode($attachment)); 
        
    $message .= "--MIME_BOUNDRY\n";
    $message .= "Content-Type: application/zip; name=\"$name\"\n";
    $message .= "Content-disposition: attachment\n";
    $message .= "Content-Transfer-Encoding: base64\n";
    $message .= "\n";
    $message .= "$attachment\n";
    $message .= "\n";
//message ends
    $message .= "--MIME_BOUNDRY--\n";

    mail($to, $subject, $message, $headers);