plugin.php

<?php
/*
Plugin Name: reCAPTCHA v3 for YOURLS Admin
Plugin URI: https://github.com/axilaris/admin-yourls-recaptcha-v3/
Description: Adds reCAPTCHA v3 to the YOURLS Admin login form.
Version: 1.0
Author: Firdaus Mah
Author URI: https://do.my/
*/

// reCAPTCHA V3 Site Key
define("recaptchaV3SiteKey", RECAPTCHAV3SITEKEY );

// reCAPTCHA V3 Secret Key
define("recaptchaV3SecretKey",  RECAPTCHAV3SECRETKEY);

// reCAPTCHA script to the head section of the HTML file
yourls_add_action('html_head', 'recaptcha_v3_html_head');
function recaptcha_v3_html_head() {
    echo '<script src="https://www.google.com/recaptcha/api.js?render=' . recaptchaV3SiteKey . '"></script>';
}

// reCAPTCHA widget to the YOURLS admin login form
yourls_add_action('login_form_bottom', 'recaptcha_v3_login_form');
function recaptcha_v3_login_form() {
    echo '<div id="recaptcha"></div>';
    echo '<input type="hidden" name="token" id="tokenInput">';
}

// Initialize reCAPTCHA widget
yourls_add_action('login_form_end', 'recaptcha_v3_inject_script');
function recaptcha_v3_inject_script() {
    echo '<script>
        grecaptcha.ready(function() {
            grecaptcha.execute(\'' . recaptchaV3SiteKey . '\', {action: \'submit\'}).then(function(token) {
                document.getElementById(\'tokenInput\').value = token;
            });
        });
    </script>';
}


// Initialize reCAPTCHA widget and verify user's response
yourls_add_action('pre_login_username_password', 'recaptcha_v3_validation');
function recaptcha_v3_validation() {
    $token = $_POST['token'];

    // call curl to POST request
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL,"https://www.google.com/recaptcha/api/siteverify");
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query(array('secret' => recaptchaV3SecretKey, 'response' => $token)));
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    $response = curl_exec($ch);
    curl_close($ch);
    $arrResponse = json_decode($response, true);

    // verify the response
    if($arrResponse["success"] == '1' && $arrResponse["score"] >= 0.5) {
        // reCAPTCHA succeeded
        return true;
    } else {
        // reCAPTCHA failed
        yourls_login_screen( $error_msg = 'reCAPTCHA verification failed' );
        yourls_die('reCAPTCHA verification failed. Please try again.');
        return false;
    }
}