Validate Select User field in PMPro Checkout

validate_select_user_field_in_pmpro_checkout.php

<?php
/**
 * Validate a custom dropdown field in PMPro checkout to ensure a valid selection.
 *
 * In this gist, if the user selects the option ("SELECT ONE"), an error message is displayed.
 * IMPORTANT - Remember you need to add this "default" option to your list of options in the select user field.
 * The script attempts to retrieve the field's label dynamically for a more informative error.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

/**
 * Validate a custom dropdown field to ensure a valid selection.
 *
 * @param bool $okay Whether registration checks pass.
 * @return bool Updated validation result.
 */
function pmpro_validate_custom_dropdown( $okay ) {
    $field_name  = 'your_dropdown_field'; // Update with actual field name.
    $field_label = '';

    // Check if the dropdown selection is invalid.
    if ( ! empty( $_POST[ $field_name ] ) && $_POST[ $field_name ] === "SELECT ONE" ) {
        // Attempt to retrieve the label dynamically.
        if ( ! empty( $_SERVER['HTTP_REFERER'] ) ) {
            $html = file_get_contents( $_SERVER['HTTP_REFERER'] );
            libxml_use_internal_errors( true ); // Suppress parsing warnings.
            $dom = new DOMDocument();
            $dom->loadHTML( $html );
            libxml_clear_errors();

            $labels = $dom->getElementsByTagName( 'label' );
            foreach ( $labels as $label ) {
                if ( $label->getAttribute( 'for' ) === $field_name ) {
                    $field_label = trim( $label->textContent );
                    break;
                }
            }
        }

        // Set a default fallback if label is not found.
        if ( empty( $field_label ) ) {
            $field_label = 'the select dropdown user field';
        }

        pmpro_setMessage( "You must select a valid option for '{$field_label}'.", "pmpro_error" );
        return false;
    }

    return $okay;
}
add_filter( 'pmpro_registration_checks', 'pmpro_validate_custom_dropdown' );