How do I POST an array with multipart/form-data encoding?

Question

In a GET parameter string, or an "x-www-form-urlencoded" POST request, it's possible to specify an array of parameters by naming them with brackets (e.g. "name[]").

Is there a "correct" (or at least a wide-spread convention) way to specify an array of parameters with a "multipart/form-data" POST request?

Would the following be correct?

Content-Type: multipart/form-data; boundary=--abc

--abc
Content-Disposition: form-data; name="name[]"

first index
--abc
Content-Disposition: form-data; name="name[]"

second index

If it varies by platform, I'm interested in the convention for Apache/PHP.

Solution

If you want an associated array you can pass index in a name of a form field:

Content-Type: multipart/form-data; boundary=--abc

--abc
Content-Disposition: form-data; name="name[first]"

first value
--abc
Content-Disposition: form-data; name="name[second]"

second value

Then on php level print_r($_POST) would give you

Array ( [name] => Array ( [first] => 'first value', [second] => 'second value' ) )

If you are after just a normal ordered array then same as you did:

Content-Type: multipart/form-data; boundary=--abc

--abc
Content-Disposition: form-data; name="name[]"

first index
--abc
Content-Disposition: form-data; name="name[]"

second index

Then on php level print_r($_POST) would give you

Array ( [name] => Array ( [0] => 'first index', [1] => 'second index' ) )

Params with [] in their names translating into arrays on a server side is a feature specific to PHP (http://www.php.net/manual/en/faq.html.php#faq.html.arrays).

As for multipart encoding you can find more in RFC: http://www.ietf.org/rfc/rfc1867.txt

Comments

[DougW]

Thanks Alexei. Do you happen to have a reference to any spec or documentation that defines this? Specifically with regard to a multipart POST vs a standard urlencoded one?

[Alexei Tenitski]

I've added couple of links to the answer

[DougW]

I'm definitely being nit-picky here, but I do want to point out that the PHP docs only mention this convention with regard to web forms which always POST via x-www-form-urlencoded. I'm guessing that by the time POST variables get handed off from Apache to PHP that there is no distinction between content types, but I have not actually read anything that confirms that. And there are definitely a lot of seemingly safe assumptions about PHP that turn out to be disastrously incorrect. I'll make sure I verify this.

[Aidan Do]

This also works similarly for node.js express servers using multer. Though name[foo] will give you an object: { foo: 'value' }. Whereas name[] or name[0] will give you an array: [ 'value' ].