check existed gravatar. exited or return default gravatar image

check-existed-avatar.php

<?php
// check gravatar existance
function check_existed_graverter($email) {

    $email_hash = md5($email);

    $url = "https://www.gravatar.com/avatar/{$email_hash}?d=404";


    $response = wp_remote_head($url);

    if (is_wp_error($response)) :
        return false;
    else :
        return ($response['response']['code'] == 200);
    endif;
}

// return initial letter according to user
function get_letter($comment) {

    if ($comment instanceof \WP_Comment) :

        $letter = mb_substr($comment->comment_author, 0, 1);

    elseif (is_string($comment) && is_email($comment)) :

        $letter = mb_substr($comment, 0, 1);

    else :

        $user = get_user_by('ID', $comment->user_id);

        if (!$user || ($user instanceof \WP_Error)) :

            $letter = mb_substr($comment, 0, 1);

        else :

            if ($user->first_name == '') :

                $letter = mb_substr($user->user_email, 0, 1);

            else :

                $letter = mb_substr($user->first_name, 0, 1);

            endif;

        endif;

    endif;

    return $letter;
}

?>