Redirect to "Thank you" page after register new user on custom register form

Question

Until now I was able to create the "Custom Register Form" on my Wordpress website, and send an activation mail to the registered user with the link for the activation.

What I want to do is soon after the registration process, the user is redirected to a "Thank you" page. But when i add to the code the wp_redirect(); it gives the error of "Cannot modify header information - headers already sent by..."

Below is the code for the "Custom Register Form"

  <?php
/*
Template Name: Register Page
*/
?>
<?php wp_head(); the_post(); ?>

      <div class="half-d" id="account-log">

            <form method="post">

        <label class="reg-form-lab p-label">Last Name</label>
        <input type="text" value="" name="last_name" id="last_name" class="reg-form-inp"/>
        <label class="reg-form-lab">First Name</label>
        <input type="text" value="" name="first_name" id="first_name" class="reg-form-inp"/>
        <label class="reg-form-lab p-label">Email</label>
        <input type="text" value="" name="email" id="email" class="reg-form-inp"/>
        <label class="reg-form-lab p-label">Username</label>
        <input type="text" value="" name="username" id="username" class="reg-form-inp"/>
        <label class="reg-form-lab p-label">Password</label>
        <input type="password" value="" name="pwd1" id="pwd1" class="reg-form-inp"/>
        <label class="reg-form-lab p-label">Password again</label>
        <input type="password" value="" name="pwd2" id="pwd2" class="reg-form-inp"/>
        <div class="alignleft"><p><?php if($sucess != "") { echo $sucess; } ?> <?php if($err != "") { echo $err; } ?></p></div>
        <button type="submit" name="btnregister" class="button" id="wp-submit" >Submit</button>
        <input type="hidden" name="task" value="register" />
    </form>

      </div>


      <div class="half-d" id="social-log">


        <p class="new-acc" id="iscr"><a href="https://mywebpage/login">Sign in</a></p>


      </div>


<?php
    $err = '';
    $success = '';

    global $wpdb, $PasswordHash, $current_user, $user_ID;

    if(isset($_POST['task']) && $_POST['task'] == 'register' ) {


        $pwd1 = $wpdb->escape(trim($_POST['pwd1']));
        $pwd2 = $wpdb->escape(trim($_POST['pwd2']));
        $first_name = $wpdb->escape(trim($_POST['first_name']));
        $last_name = $wpdb->escape(trim($_POST['last_name']));
        $email = $wpdb->escape(trim($_POST['email']));
        $username = $wpdb->escape(trim($_POST['username']));

        if( $email == "" || $pwd1 == "" || $pwd2 == "" || $username == "" || $first_name == "" || $last_name == "") {
            $err = 'Please don\'t leave the required fields.';
        } else if(!filter_var($email, FILTER_VALIDATE_EMAIL)) {
            $err = 'Invalid email address.';
        } else if(email_exists($email) ) {
            $err = 'Email already exist.';
        } else if($pwd1 <> $pwd2 ){
            $err = 'Password do not match.';
        } else {

            $user_id = wp_insert_user( array ('first_name' => apply_filters('pre_user_first_name', $first_name), 'last_name' => apply_filters('pre_user_last_name', $last_name), 'user_pass' => apply_filters('pre_user_user_pass', $pwd1), 'user_login' => apply_filters('pre_user_user_login', $username), 'user_email' => apply_filters('pre_user_user_email', $email), 'role' => 'subscriber' ) );
            if( is_wp_error($user_id) ) {
                $err = 'Error on user creation.';
            } else {
                do_action('user_register', $user_id);
                $success = 'You\'re successfully register';
            }

        }
        //wp_redirect('xxxxxxxxxxxxxxxxxx');
    }
?>

And the code that I insert into functions.php file in order to send the activation email is below:

add_action( 'user_register', 'actuser_user_register' );
function actuser_user_register( $user_id ) {

$user = get_user_by('id',$user_id);

$user_email = stripslashes($user->user_email);

 $code = sha1( $user_id . time() );

update_user_meta( $user_id, 'activation_key', $code );
update_user_meta( $user_id, 'activation_flag', 0 );

$activation_link = add_query_arg( array( 'key' => $code, 'user' => $user_id ), get_permalink($id));

$firstName = get_user_meta($user->ID, "first_name", true);
$lastName = get_user_meta($user->ID, "last_name", true);

$to = $user_email;
$subject = 'Activation Required';
$message = 'xxxxxxxxxxxxxxx';


wp_mail( $to, $subject, $message);
//wp_redirect('xxxxxxxxxx');
}

Answer 1

OK, so it's clear that the redirection won't work in this case. You can't output any HTML before sending headers, and redirection sends a header.

The easiest (but bad) solution is to move you PHP code to the top of that page template:

<?php
    /* Template Name: Register Page */

    $err = '';
    $success = '';

    global $wpdb, $PasswordHash, $current_user, $user_ID;

    if(isset($_POST['task']) && $_POST['task'] == 'register' ) {

        ...
        wp_redirect('xxxxxxxxxxxxxxxxxx');
        die();
    }

    wp_head(); the_post();
?>
  ...
  <div class="half-d" id="account-log">

        <form method="post">
        ...
        </form>

  </div>
  ...

You also have to die the script after doing redirection.

But... This isn't very good way to do this. You should not process any forms directly in templates - templates are for displaying site and not for processing any data and any logic.

Much nicer way would be to send that form to wp_admin_url('admin-post.php)' and processing it using [admin_post_(action)`]1.

Answer 2

Use this hook for redirect to another page after registers user.

add_action( 'user_register', 'thankyou_page_redirect', 10, 1 );

function thankyou_page_redirect( $user_id ) {

    $thankyoupage  = site_url('thank-you');

    wp_redirect($thankyoupage);
    exit;

}