headers in SSE request

I'm currently setting up a messaging system with PHP. To realize the "instant" messaging system, I use a unidirectional connection with the server, the SSE. But for my SSE to work, I send data via the url with a http_build_query: [code JS]

const eventSource = new EventSource("http://localhost/sse_test.php?<?= http_build_query($url) ?>");

Except that I quickly realized that it will not work indefinitely because in my system, the amount of data sent is proportional to the number of conversations and groups: [code PHP]

$url = [];
for ($i = 0; $i < $num; $i++) {
    $tableau_url = Array(
        "conv" => $conv_table_sse[$i],
        "to" => $to_table_sse[$i]
    );
    array_push($url, $tableau_url);     
}

Except that the maximum length of a url is 2048 characters in general. So I did some research and I saw that you can add headers for an SSE connection. So here is what I did: [code JS]

var headers = JSON.parse('<?= json_encode($url) ?>');
const eventSource = new EventSource("http://localhost/sse_test.php",{ 
withCredentials: true, // allows to send the cookies with the request
headers: headers // add headers to the request
});

Finally I will have to recover my data with this code: [code PHP]

(I didn't put all the code because it doesn't influence this one because before it worked very well with $_GET)

if (isset($_SERVER)) {
    if (!empty($_SERVER)) {
        $parametres = $_SERVER;

Except that I have a problem when retrieving the data. It is as if the data I was passing through the headers did not exist, here is a screen of the var_dump($_SERVER) :

And this is what should be there:

After explaining my situation to you, I hope that you will be able to enlighten me and explain the errors that I may have inadvertently made.

source https://stackoverflow.com/questions/75784462/headers-in-sse-request