Understanding and Troubleshooting CMB2 Sanitization Callbacks

If you've ever built a WordPress site with custom metaboxes, you've likely used the powerful CMB2 library. A common point of confusion and troubleshooting for developers is how CMB2 handles data sanitization, particularly with custom callback functions. This guide will explain how sanitization works in CMB2 and provide solutions to common issues you might encounter.

What is Sanitization in CMB2?

Sanitization is the process of cleaning and validating user-submitted data before it's saved to the database. CMB2 includes robust built-in sanitization for all field types, but also provides the flexibility to define your own custom sanitization callbacks using the sanitization_cb parameter.

Common Sanitization Issues and Solutions

1. Sanitized Value Doesn't Appear Immediately After Save

Problem: You've implemented a custom sanitization_cb that modifies the field value (e.g., trimming text), but after saving the post, the field still displays the original unsanitized value until you reload the page.

Why This Happens: This behavior is typically related to how WordPress handles post data caching and display after submission. The field might be showing the submitted POST data rather than the sanitized value retrieved from the database.

Solution: This is generally expected behavior in the WordPress admin interface. The sanitized value is properly saved to the database, and will display correctly on subsequent page loads.

2. Custom Sanitization Callback Not Firing

Problem: You've defined a sanitization_cb within a PHP class, but the function never gets called when saving field data.

Why This Happens: This is often a scoping issue. If your callback method is defined as protected or private within a class, CMB2 cannot access it.

Solution: Ensure your sanitization callback method is declared as public within your class:

public function sanitize_css_classes( $value, $field_args, $field ) {
    // Your sanitization logic here
    return $value;
}

3. Empty Values Are Stripped from Sanitized Data

Problem: When using get_sanitized_values() method, empty field values are completely removed from the returned array, making it impossible to save empty values to clear existing data.

Why This Happens: This is by design in CMB2's sanitization process, which filters out empty values to reduce unnecessary database operations.

Solution: If you need to preserve empty values, you may need to implement custom handling that processes the raw POST data directly rather than relying solely on get_sanitized_values().

4. Validating Field Values Against Specific Criteria

Problem: You need to validate that a numeric field doesn't exceed a certain value (e.g., not above 30) and show an error message when validation fails.

Solution: While CMB2 doesn't have built-in validation messaging, you can implement WordPress admin notices:

function validate_numeric_field( $value, $field_args, $field ) {
    if ( $value > 30 ) {
        // Queue admin error message
        add_action( 'admin_notices', function() {
            echo '<div class="error"><p>Value must not exceed 30</p></div>';
        });
        // Return original value to prevent saving
        return $value;
    }
    return $value;
}

Best Practices for CMB2 Sanitization

• Always use appropriate WordPress sanitization functions like sanitize_text_field(), sanitize_html_class(), etc., within your callbacks.
• For frontend forms, remember to sanitize $_POST values before re-populating fields to prevent security issues.
• Test your sanitization callbacks thoroughly with various input values, including edge cases.
• Consider using the cmb2_override_{field_id}_meta_save filter for complex sanitization scenarios that involve multiple fields.

By understanding how CMB2 handles sanitization and implementing these solutions, you can ensure your custom metabox data is clean, secure, and behaves exactly as expected.