GMail API - Getting certain message headers or fields - php

Question

I have successfully connected GMail API via G Suite account and service account. I can get a message list and I can retrieve messages by IDs. I'm working with PHP.
What I'm having problems with is to get for example the FROM or TO headers, SUBJECT or the snippet field.
$optParam = array('format' => 'metadata', 'metadataHeaders'=>['subject','from'], 'fields'=>['snippet','labelIds']);
$fullMessage = $service->users_messages->get($user, $id, $optParam);
This will return the snippet, but not the subject or from or the labelIds.
If I use the GMail "Try this API" and use the id of the message and use "snippet" in the "fields" entry, I just get the snippet back as:
{
"snippet": "Short snippet of the message"
}
If I use:
$optParam = array('format' => 'metadata', 'metadataHeaders'=>['subject','from','to']);
I do get the 3 headers, but I also get a lot more information, including the labels and snippet - about 3K for each message.
I just can't seem to be able to specify a small subset of the data. All I need is to show messages as a list with the subject, date/time, from/to.
I don't care so much about the amount of data, but it takes on average about 3.5 seconds to retrieve the data for just 14 message!
Is there a way to restrict this so I don't get all the "extra" data or speed the retrieval up somehow?

Answer 1

Sending the request would involve specifying the metadata keys as well as the parameter names of the fields you want to obtain. You can use an HTTP GET request with the URI to get the ‘to’, ‘from’, ‘subject’ and ‘snippet’ with https://www.googleapis.com/gmail/v1/users/me/messages/<MESSAGE_ID>?format=metadata&metadataHeaders=to&metadataHeaders=from&metadataHeaders=subject&fields=snippet%2C+payload%2Fheaders, which will also limit the headers you obtain.
In PHP you can use this:
$optParam = array('format' => 'metadata', 'metadataHeaders'=>['subject', 'from', 'to'], 'fields'=>'payload/headers,snippet');
Note that the fields parameter needs to be sent as a string and not an array.
Also be aware there is a known issue with the Gmail API where using the https://www.googleapis.com/auth/gmail.metadata scope will not return the snippet.
You’ll need to use https://www.googleapis.com/auth/gmail.readonly instead.
You can also make a batch of requests in one network call which will help speed up overall execution time as documented here.