PHPのmultipartでHTML形式のメッセージを受ける

前提・実現したいこと

自分、あまりPHPには詳しくないですが、iPhoneから遠隔サーバを通してメールを送るためのSendmailのコードを書いています。メールには複数の添付書類をつけることになっているので、multipartのContent-Typeを指定しています。以下のコードがそのPHPのコードです。

該当のソースコード

PHP
1<?php
2   error_reporting(E_ALL);
3   ini_set('display_errors', 1);
4   mb_internal_encoding ("UTF-8");
5       
6   if(isset($_FILES) && (bool) $_FILES) {
7       $files = array();
8       foreach($_FILES as $name=>$file) {
9           $file_name = $file['name'];
10           $temp_name = $file['tmp_name'];
11           $file_type = $file['type'];
12           $path_parts = pathinfo($file_name);
13           $ext = $path_parts['extension'];
14           array_push($files, $file);
15       }
16       
17       // email fields: to, from, subject, and so on
18       $to = $_POST['mail_recipient'];
19       $cc = $_POST['mail_copy'];
20       $bcc = $_POST['mail_bcc'];
21       $from = $_POST['mail_sender'];
22       $myReturn = $_POST['mail_return'];
23       $subject = $_POST['mail_subject'];
24       $message = $_POST['mail_message'];
25       $returnpath = "-f" . $myReturn;
26       $headers = "From: $from\r\n";
27       $headers .= "Cc: $cc\r\n";
28       $headers .= "Bcc: $bcc";
29       
30       // boundary
31       $semi_rand = md5(time());
32       $mime_boundary = "==Multipart_Boundary_x{$semi_rand}x";
33       
34       // headers for attachment
35       $headers .= "\nMIME-Version: 1.0\n" . "Content-Type: multipart/mixed;\n" . " boundary=\"{$mime_boundary}\"";
36       
37       // multipart boundary
38       $message = "This is a multi-part message in MIME format.\n\n" . "--{$mime_boundary}\n" . "Content-Type: text/plain;\n" . "Content-Transfer-Encoding: 7bit\n\n" . $message . "\n\n";
39       $message .= "--{$mime_boundary}\n";
40       
41       // preparing attachments
42       for($x=0; $x<count($files); $x++){
43           $file = fopen($files[$x]['tmp_name'], "rb");
44           $data = fread($file, filesize($files[$x]['tmp_name']));
45           fclose($file);
46           $data = chunk_split(base64_encode($data));
47           $name = $files[$x]['name'];
48           $message .= "Content-Type: {\"application/octet-stream\"};\n" . " name=\"$name\"\n" .
49           "Content-Disposition: attachment;\n" . " filename=\"$name\"\n" .
50           "Content-Transfer-Encoding: base64\n\n" . $data . "\n\n";
51           $message .= "--{$mime_boundary}\n";
52       }
53       
54       // send
55       $ok = mail($to, $subject, $message, $headers, $returnpath);
56       if ($ok) {
57           echo "<p>mail sent to $to!</p>";
58       } else {
59           echo "<p>mail could not be sent!</p>";
60       }
61   } else {
62       // email fields: to, from, subject, and so on
63       $to = $_POST['mail_recipient'];
64       $cc = $_POST['mail_copy'];
65       $bcc = $_POST['mail_bcc'];
66       $from = $_POST['mail_sender'];
67       $myReturn = $_POST['mail_return'];
68       $subject = $_POST['mail_subject'];
69       $message = $_POST['mail_message'];
70       $returnpath = "-f" . $myReturn;
71       $headers = "From: $from\r\n";
72       $headers .= "Cc: $cc\r\n";
73       $headers .= "Bcc: $bcc";
74       
75       // send
76       $ok = mail($to, $subject, $message, $headers, $returnpath);
77       if ($ok) {
78           echo "<p>mail sent to $to!</p>";
79       } else {
80           echo "<p>mail could not be sent!</p>";
81       }
82   }
83?>

それで上のコードではメッセージの本文はPlain Textなんですが、それをHTML形式のテキストを送りたいのですが、どうしたらいいでしょうか？このURLにある説明を見ると、

PHP
1$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";

とContent-typeを「text/html」にしろ、とあります。ただ自分の上のコードではすでにContent-typeを「multipart/mixed」と指定しています。すると両方を指定することはできるのでしょうか？
よろしくお願いします。

更新1

どうも自分が誤解していたようです。自分のPHPのコードで

php
1$message = "This is a multi-part message in MIME format.\n\n" . "--{$mime_boundary}\n" . "Content-Type: text/plain;\n" . "Content-Transfer-Encoding: 7bit\n\n" . $message . "\n\n";

とあるので、ここを「text/html」に変えればいいんでしょうね。ただこのURLにあるサイトによるとPlain TextとHTML Textの両方を送る、とあります。

更新2

その後以下のように「Content-Type」を「text/html」に変更してみました。でもメールの内容はPlain Textで送られます。何がいけないのでしょうか？

PHP
1<?php
2   error_reporting(E_ALL);
3   ini_set('display_errors', 1);
4   mb_internal_encoding ("UTF-8");
5       
6   if(isset($_FILES) && (bool) $_FILES) {
7       $files = array();
8       foreach($_FILES as $name=>$file) {
9           $file_name = $file['name'];
10           $temp_name = $file['tmp_name'];
11           $file_type = $file['type'];
12           $path_parts = pathinfo($file_name);
13           $ext = $path_parts['extension'];
14           array_push($files, $file);
15       }
16       
17       // email fields: to, from, subject, and so on
18       $to = $_POST['mail_recipient'];
19       $cc = $_POST['mail_copy'];
20       $bcc = $_POST['mail_bcc'];
21       $from = $_POST['mail_sender'];
22       $myReturn = $_POST['mail_return'];
23       $subject = $_POST['mail_subject'];
24       $message = $_POST['mail_message'];
25       $htmlMessage = $_POST['mail_html'];
26       $returnpath = "-f" . $myReturn;
27       $headers = "From: $from\r\n";
28       $headers .= "Cc: $cc\r\n";
29       $headers .= "Bcc: $bcc";
30       
31       // boundary
32       $semi_rand = md5(time());
33       $mime_boundary = "==Multipart_Boundary_x{$semi_rand}x";
34       
35       // headers for attachment
36       $headers .= "\nMIME-Version: 1.0\n" . "Content-Type: multipart/mixed;\n" . " boundary=\"{$mime_boundary}\"";
37       
38       // multipart boundary
39       $message = "This is a multi-part message in MIME format.\n\n" . "--{$mime_boundary}\n" . "Content-Type: text/html;\n" . "Content-Transfer-Encoding: 7bit\n\n" . $htmlMessage . "\n\n";
40       $message .= "--{$mime_boundary}\n";
41       
42       // preparing attachments
43       for($x=0; $x<count($files); $x++){
44           $file = fopen($files[$x]['tmp_name'], "rb");
45           $data = fread($file, filesize($files[$x]['tmp_name']));
46           fclose($file);
47           $data = chunk_split(base64_encode($data));
48           $name = $files[$x]['name'];
49           $message .= "Content-Type: {\"application/octet-stream\"};\n" . " name=\"$name\"\n" .
50           "Content-Disposition: attachment;\n" . " filename=\"$name\"\n" .
51           "Content-Transfer-Encoding: base64\n\n" . $data . "\n\n";
52           $message .= "--{$mime_boundary}\n";
53       }
54       
55       // send
56       $ok = mail($to, $subject, $message, $headers, $returnpath);
57       if ($ok) {
58           echo "<p>mail sent to $to!</p>";
59       } else {
60           echo "<p>mail could not be sent!</p>";
61       }
62   } else {
63       // email fields: to, from, subject, and so on
64       $to = $_POST['mail_recipient'];
65       $cc = $_POST['mail_copy'];
66       $bcc = $_POST['mail_bcc'];
67       $from = $_POST['mail_sender'];
68       $myReturn = $_POST['mail_return'];
69       $subject = $_POST['mail_subject'];
70       $message = $_POST['mail_message'];
71       $returnpath = "-f" . $myReturn;
72       $headers = "From: $from\r\n";
73       $headers .= "Cc: $cc\r\n";
74       $headers .= "Bcc: $bcc";
75       
76       // send
77       $ok = mail($to, $subject, $message, $headers, $returnpath);
78       if ($ok) {
79           echo "<p>mail sent to $to!</p>";
80       } else {
81           echo "<p>mail could not be sent!</p>";
82       }
83   }
84?>

行動規範の内容に同意します

回答1件

自己解決

添付ファイルがない場合の部分に以下のものを挿入することで解決しました。

PHP
1// boundary
2$semi_rand = md5(time());
3$mime_boundary = "==Multipart_Boundary_x{$semi_rand}x";
4// headers for attachment
5$headers .= "\nMIME-Version: 1.0\n" . "Content-Type: multipart/mixed;\n" . " boundary=\"{$mime_boundary}\"";
6// multipart boundary
7$message = "This is a multi-part message in MIME format.\n\n" . "--{$mime_boundary}\n" . "Content-Type: text/html;\n" . "Content-Transfer-Encoding: 7bit\n\n" . $htmlMessage . "\n\n";
8$message .= "--{$mime_boundary}\n";