Handling error states with admin_post

I have a front-end form posting to admin_post. Here, I do some validation. If there are errors, I want to show an error message for the relevant fields.

However, I don’t know how to redirect back to the submission page and retain the field values.

Currently I’m using the below:.

add_action( 'admin_post_form', 'form_post' );
function form_post() {
  $validate = // Validation functions go here
  if ($validate == 'error') {
    $url = wp_get_referer();
    $url = add_query_arg( 'error', 'email', $url );
    wp_redirect( $url );
    die();
  }
}

But this (naturally) doesn’t retain the filled form field values, so the user has to start all over again.

If I were posting to the same page as the form this would be simple – just set the value as <input value="<?php echo $_POST['email']; ?>"> but I’m not sure how to achieve the same effect using admin-post.

Can you help?

You can use set_transient with the user_id if you are relying on users being logged in, otherwise, cookie would probably be better. For logged in users, you can have the user_id get set within the name of the transient. An example here:

function form_post() {
    $user_info = wp_get_current_user();
    $user_id = $user_info->exists() && !empty($user_info->ID) ? $user_info->ID : 0;
    $url = wp_get_referer();

    // Validate the form fields and populate this array when errors occur
    $validation_errors = array();  // I like to use the id of the elements as keys and the error string as the values of the array, so i can add error class to the elements if needed easily...

    // Now to save the transient...
    if (!empty($validation_errors)) {
        set_transient('validation_errors_' . $user_id, $my_errors);
        wp_redirect($url);
        die();
    }
}

Now before you output your form element on your page, you just need to check the transient for that user, if exists, output the errors, than delete the transient right after that…

For example:

<?php 
$user_info = wp_get_current_user();

$user_id = $user_info->exists() && !empty($user_info->ID) ? $user_info->ID : 0;
$validation_errors = get_transient('validation_errors_' . $user_id);

if ($validation_errors !== FALSE) {
   echo '<div class="errors">Please fix the following Validation Errors:<ul><li>' . implode('</li><li>', $validation_errors) . '</li></ul></div>';
   delete_transient('validation_errors_' . $user_id);
} ?>

<form ...>

</form>