Google API Contacts v.3, PHP: add a contact to the group

Question

I successfully managed to create a new contact with cURL, but when I want to add group membership for this contact, I get 400 error. I've read this docs and made the same request, but it didn't work. What am I doing wrong? Thanks for any ideas!

This is how I create an XML with group information:

$doc = new DOMDocument('1.0', 'UTF-8');
$doc->formatOutput = true;

$entry = $doc->createElement('entry');
$entry->setAttribute('gd:etag', $etag);
$doc->appendChild($entry);

$category = $doc->createElement('category');
$category->setAttribute('scheme', 'http://schemas.google.com/g/2005#kind');
$category->setAttribute('term', 'http://schemas.google.com/contact/2008#contact');
$entry->appendChild($category);

$id = $doc->createElement('id', 'http://www.google.com/m8/feeds/contacts/default/base/'.$idgmail);
$entry->appendChild($id);

$updated = $doc->createElement('updated', $update_info);
$entry->appendChild($updated);

// Add group info (My Contacts)

$group = $doc->createElement('gContact:groupMembershipInfo');
$entry->appendChild($group);
$group->setAttribute('deleted', 'false');
$group->setAttribute('href', 'http://www.google.com/m8/feeds/groups/default/base/6');

// Add another group info

$group = $doc->createElement('gContact:groupMembershipInfo');
$entry->appendChild($group);
$group->setAttribute('deleted', 'false');
$group->setAttribute('href', 'http://www.google.com/m8/feeds/groups/default/base/'.$my_group_id);

$group_info = $doc->saveXML();

And this is my cURL:

$url = 'https://www.google.com/m8/feeds/contacts/default/full/'.$idgmail.'/';

$headers = array(
'Host: www.google.com',
'Gdata-version: 3.0',
'Content-length: '.strlen($group_info),
'Content-type: application/atom+xml',
'If-Match: *',
'Authorization: OAuth '.$access,
);

$curl = curl_init();

curl_setopt($curl, CURLOPT_URL, $url);
curl_setopt($curl, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_POSTFIELDS, $group_info);
curl_setopt($curl, CURLOPT_FAILONERROR, true);

$resp = curl_exec($curl); 
print_r($resp); // Prints nothing
echo curl_getinfo($curl, CURLINFO_HTTP_CODE); // Gives 400
curl_close($curl);

Answer 1

OK, I've figured it out by myself:)
1) First of all, to update a contact you should use PUT request, not POST.
2) In your XML you can't use "default" (you'll get another error), you should use the full email address:

$group = $doc->createElement('gContact:groupMembershipInfo');
$entry->appendChild($group);
$group->setAttribute('deleted', 'false');
$group->setAttribute('href', 'http://www.google.com/m8/feeds/groups/user.email@gmail.com/base/6');  

3) You will get 400 error if you haven't specified the namespace gContact. The whole thing for the entry tag should look like this:

$entry = $doc->createElement('entry');
$entry->setAttribute('xmlns', 'http://www.w3.org/2005/Atom');
$entry->setAttribute('xmlns:gd', 'http://schemas.google.com/g/2005');
$entry->setAttribute('xmlns:gContact', 'http://schemas.google.com/contact/2008');
$doc->appendChild($entry); 

4) Finally, to add a contact to a particular group, you don't need to update it (as I thought from the docs), you can do it while creating a contact (yes, now it seems obvious). If you try to update a contact's group without creating it first, you'll get 400 error (Entry does not have any fields set).
Hope this will help somebody!
P.S. To solve these problems I used "OAuth 2.0 Playground" by Google, very useful! https://developers.google.com/oauthplayground/