Add reCAPTCHA to WordPress without plugins

1. Add reCAPTCHA to WordPress Comment Section
2. Add reCaptcha on WordPress Admin Login Page Without Plugin
3. Add reCaptcha to WordPress Lost Password Form Without Plugin
4. Add reCaptcha to WordPress registration Form Without Plugin

Addd this code first on function.php

function my_recaptcha_key(){
	$sitekey= "6LekREUpAAAAAAFoHs2YZIPfAJin4twqr_PAkRF4";
	$secretkey= "6LekREUpAAAAAFOjI7-7ZgqKKBbV7k3V5IxENZz7";
	return explode(",", $sitekey.",".$secretkey ); 	
}

1. Add reCAPTCHA to WordPress Comment Section

/*
*Add reCAPTCHA to WordPress Comment Section
*/
add_action( 'wp_head', function(){
wp_enqueue_script('google-recaptcha', 'https://www.google.com/recaptcha/api.js');
} );
function add_google_recaptcha($submit_field) {
    $submit_field['submit_field'] = '<div class="g-recaptcha" data-sitekey="'.my_recaptcha_key()[0].'"></div><br>' . $submit_field['submit_field'];
    return $submit_field;
}
if (!is_user_logged_in()) {
    add_filter('comment_form_defaults','add_google_recaptcha');
}
function is_valid_captcha($captcha) {
$captcha_postdata = http_build_query(array(
                            'secret' => my_recaptcha_key()[1],
                            'response' => $captcha,
                            'remoteip' => $_SERVER['REMOTE_ADDR']));
$captcha_opts = array('http' => array(
                      'method'  => 'POST',
                      'header'  => 'Content-type: application/x-www-form-urlencoded',
                      'content' => $captcha_postdata));
$captcha_context  = stream_context_create($captcha_opts);
$captcha_response = json_decode(file_get_contents("https://www.google.com/recaptcha/api/siteverify" , false , $captcha_context), true);
if ($captcha_response['success'])
    return true;
else
    return false;
}
 
function verify_google_recaptcha() {
$recaptcha = $_POST['g-recaptcha-response'];
if (empty($recaptcha))
    wp_die( __("<b>ERROR:</b> please select <b>I'm not a robot!</b><p><a href='javascript:history.back()'>« Back</a></p>"));
else if (!is_valid_captcha($recaptcha))
    wp_die( __("<b>Go away SPAMMER!</b>"));
}
if (!is_user_logged_in()) {
    add_action('pre_comment_on_post', 'verify_google_recaptcha');
}

or Add reCaptcha on WordPress Admin Login Page Without Plugin

/*
*Add reCaptcha on WordPress Admin Login Page Without Plugin
*/

function login_style() {
    wp_register_script('login-recaptcha', 'https://www.google.com/recaptcha/api.js', false, NULL);
    wp_enqueue_script('login-recaptcha');
	echo "<style>p.submit, p.forgetmenot {margin-top: 10px!important;}.login form{width: 303px;} div#login_error {width: 322px;}</style>";
}
add_action('login_enqueue_scripts', 'login_style');
function add_recaptcha_on_login_page() {
    echo '<div class="g-recaptcha brochure__form__captcha" data-sitekey="'.my_recaptcha_key()[0].'"></div>';
}
add_action('login_form','add_recaptcha_on_login_page');
function captcha_login_check($user, $password) {
    if (!empty($_POST['g-recaptcha-response'])) {
        $secret = my_recaptcha_key()[1];
        $ip = $_SERVER['REMOTE_ADDR'];
        $captcha = $_POST['g-recaptcha-response'];
        $rsp = file_get_contents('https://www.google.com/recaptcha/api/siteverify?secret=' . $secret . '&response=' . $captcha .'&remoteip='. $ip);
        $valid = json_decode($rsp, true);
        if ($valid["success"] == true) {
            return $user;
        } else {
            return new WP_Error('Captcha Invalid', __('<center>Captcha Invalid! Please check the captcha!</center>'));
        }
    } else {
        return new WP_Error('Captcha Invalid', __('<center>Captcha Invalid! Please check the captcha!</center>'));
    }
}
add_action('wp_authenticate_user', 'captcha_login_check', 10, 2);

Add reCaptcha to WordPress Lost Password Form Without Plugin

/*
*Add reCaptcha to Lost Password Form Without Plugin
*/
function recaptcha() {
    echo '<script src="https://www.google.com/recaptcha/api.js" async defer></script>';
    echo '<div class="g-recaptcha brochure__form__captcha" data-sitekey="'.my_recaptcha_key()[0].'"></div>';
	echo '<style>p.message {width: 324px;}</style>';
}
add_action('lostpassword_form', 'recaptcha');

function captcha_lostpassword_check($errors) {
    if (!empty($_POST['g-recaptcha-response'])) {
        $secret = my_recaptcha_key()[1];
        $ip = $_SERVER['REMOTE_ADDR'];
        $captcha = $_POST['g-recaptcha-response'];
        $rsp = file_get_contents('https://www.google.com/recaptcha/api/siteverify?secret=' . $secret . '&response=' . $captcha .'&remoteip='. $ip);
        $valid = json_decode($rsp, true);
        if ($valid["success"] == true) {
           
        } else {
            $errors->add('Captcha Invalid', __('Captcha Invalid! Please check the captcha!'));
        }
    } else {
        $errors->add('Captcha Invalid', __('Captcha Invalid! Please check the captcha!'));
    }
}
add_action( 'lostpassword_post', 'captcha_lostpassword_check', 10, 1 );

Add reCaptcha to WordPress registration Form Without Plugin

/*
*Protect WordPress Site Registration Form with reCaptcha
*/
function recaptcha_register_form() {
    echo '<script src="https://www.google.com/recaptcha/api.js" async defer></script>';
    echo '<div class="g-recaptcha brochure__form__captcha" data-sitekey="'.my_recaptcha_key()[0].'"></div>';
	echo'<style>p.message.register {
    width: 324px;
}</style>';
}
add_action('register_form', 'recaptcha_register_form');

function captcha_registration_check($errors, $user_login, $user_email) {
    if (!empty($_POST['g-recaptcha-response'])) {
        $secret = my_recaptcha_key()[1];
        $ip = $_SERVER['REMOTE_ADDR'];
        $captcha = $_POST['g-recaptcha-response'];
        $rsp = file_get_contents('https://www.google.com/recaptcha/api/siteverify?secret=' . $secret . '&response=' . $captcha .'&remoteip='. $ip);
        $valid = json_decode($rsp, true);
        if ($valid["success"] == true) {
            return $errors;
        } else {
            return new WP_Error('Captcha Invalid', __('Captcha Invalid! Please check the captcha!'));
        }
    } else {
        return new WP_Error('Captcha Invalid', __('Captcha Invalid! Please check the captcha!'));
    }
}
add_action('registration_errors', 'captcha_registration_check', 10, 3);