Validation Hooks (PHP)

English version coming soon

This page has not been fully translated yet. See the Japanese version for the full reference and examples.

Form Plant exposes WordPress filter hooks for customizing server-side validation — replace error messages, add per-field checks, or call external APIs from PHP.

Client vs server

Use JavaScript customization for instant in-browser feedback (UX). Use the PHP hooks on this page for trustworthy validation (security). Client-side checks can be bypassed via DevTools, so server-side validation must always run independently.

Hook Reference

Hook	Purpose
fplant_before_validation	Normalize submitted data before validation
fplant_validation_required_message	Customize "required" error message
fplant_validate_field_{$field_name}	Override validation for a specific field
fplant_validation_message_email	Customize email format error message
fplant_validation_message_url	Customize URL format error message
fplant_validation_message_tel	Customize tel format error message
fplant_validation_message_number	Customize number / min / max error message
fplant_validate_field_type_{$field_type}	Add extra validation for a field type
fplant_validation_message_min_length	Customize min length error message
fplant_validation_message_max_length	Customize max length error message
fplant_validation_message_pattern	Customize regex pattern error message
fplant_validate_custom_rules	Add custom validation rule keys
fplant_validation_message_file	Customize file upload error message
fplant_validation_errors	Final adjustment of the full error array

Example: Restrict an Email Field to a Company Domain

add_filter( 'fplant_validate_field_company_email', function( $error, $field, $value, $data, $form_id ) {
    $allowed_domains = array( 'example.com', 'company.co.jp' );

    if ( ! empty( $value ) && is_email( $value ) ) {
        $domain = substr( strrchr( $value, '@' ), 1 );

        if ( ! in_array( $domain, $allowed_domains, true ) ) {
            return 'Please use your company email (@example.com or @company.co.jp).';
        }
    }

    // Return null to also run the standard validation.
    return $error;
}, 10, 5 );

Example: Add a Custom Rule (Minimum Age)

add_filter( 'fplant_validate_custom_rules', function( $error, $field, $value, $validation ) {
    if ( isset( $validation['age_restriction'] ) && ! empty( $value ) ) {
        $min_age   = intval( $validation['age_restriction'] );
        $input_age = intval( $value );

        if ( $input_age < $min_age ) {
            return sprintf( 'You must be %s or older to apply.', $min_age );
        }
    }

    return $error;
}, 10, 4 );

Notes

• Return false for "no error", a string for an error message. Avoid returning null / true (except for fplant_validate_field_{$field_name}).
• Cache external API lookups via the WordPress Transient API to avoid hitting rate limits on every submission.
• Always sanitize input (sanitize_text_field()) and escape (esc_html()) when including user input in error messages.

For more examples (i18n, file uploads, postal code validation, data normalization), see the Japanese version linked at the top.