method="post" enctype="text/plain" are not compatible?

[Revised]

The answer is, because PHP doesn't handle it (and it is not a bug):

https://bugs.php.net/bug.php?id=33741

Valid values for enctype in <form> tag are:

application/x-www-form-urlencoded
multipart/form-data

The first is the default, the second one you need only when you upload files.

@Alohci provided explanation why PHP doesn't populate $_POST array, but store the value inside a variable $HTTP_RAW_POST_DATA.

Example of what can go wrong with text/plain enctype:

file1.php:

<form method="post" enctype="text/plain" action="file2.php">
<textarea name="input1">abc
input2=def</textarea>
<input name="input2" value="ghi" />
<input type="submit">
</form>

file2.php:

<?php
print($HTTP_RAW_POST_DATA);
?>

Result:

input1=abc
input2=def
input2=ghi

No way to distinguish what is the value of input1 and input2 variables. It can be

• input1=abc\r\ninput2=def, input2=ghi, as well as
• input1=abc, input2=def\r\ninput2=ghi

No such problem when using the other two encodings mentioned before.

The difference between GET and POST:

• in GET, the variables are part of URL and are present in URL as query string, therefore they must be URL-encoded (and they are, even if you write enctype="text/plain" - it just gets ignored by the browser; you can test it using Wireshark to sniff the request packets),
• when sending POST, the variables are not part of URL, but are sent as the last header in HTTP request (POSTDATA), and you can choose whether you want to send them as text/plain or application/x-www-form-urlencoded, but the second one is the only non-ambiguous solution.

HTML5 does define how to format form data submitted as text/plain here: https://w3c.github.io/html/sec-forms.html#plain-text-form-data.

At the bottom of that section, it says:

Payloads using the text/plain format are intended to be human readable. They are not reliably interpretable by computer, as the format is ambiguous (for example, there is no way to distinguish a literal newline in a value from the newline at the end of the value).

So it not unreasonable that PHP does not attempt to interpret it and only makes it available in raw form. To me, that seems the correct approach.