Can I get my subscriber list with a GET request only?

Here is a cURL example (Add this function and append _curl to all your file_get_contents.)

<?php
/* file_get_contents replaced with curl function - Add this function and replace any use of file_get_contents with file_get_contents_curl */

function file_get_contents_curl($url) {
    $curlHeader = array(
        "Client-ID: XXXXXXXXXXXXXXXXXXXXXXXX",    /* SET CLIENT ID HERE */
        "Accept: application/vnd.twitchtv.v5+json",
        "Authorization: OAuth xxxxxxxxxxxxxxxxxxxxxxxxxxx"
    );
    $ch         = curl_init();
    curl_setopt($ch, CURLOPT_AUTOREFERER, TRUE);
    curl_setopt($ch, CURLOPT_HEADER, 0);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_FOLLOWLOCATION, TRUE);
    curl_setopt($ch, CURLOPT_HTTPHEADER, $curlHeader);
    $data = curl_exec($ch);
    curl_close($ch);
    return $data;
}
?>