Apply Bootstrap 4 custom checkboxes and radio buttons to Gravity Fields generated forms

add_bootstrap_control_class.php

<?php

// Apply Bootstrap 4 custom checkboxes and radio buttons to Gravity Fields generated forms
// Derived from https://jayhoffmann.com/using-gravity-forms-bootstrap-styles/

add_filter( 'gform_field_container', 'add_bootstrap_control_class', 10, 6 );
function add_bootstrap_control_class( $field_container, $field, $form, $css_class, $style, $field_content ) {
    $id = $field->id;
    $field_id = is_admin() || empty( $form ) ? "field_{$id}" : 'field_' . $form['id'] . "_$id";

    $control = '';
    if (strpos($field_content, 'ginput_container_checkbox') > 0) {
        $control = 'checkbox';
    } elseif (strpos($field_content, 'ginput_container_radio') > 0) {
        $control = 'radio';
    }

    // find the control class, add custom-control custom-checkbox/radio to it
    $to_find = "gchoice_";
    $new_content = str_replace($to_find, "custom-control custom-{$control} " . $to_find, $field_content);

    // did it match?
    if ($field_content != $new_content) {
        // add the classes to the label and the control

        $patterns = array();
        $patterns[0] = "/(id='choice_)/";
        $patterns[1] = "/(id='label_)/";
        $replacements = array();
        $replacements[0] = "class='custom-control-input' $1";
        $replacements[1] = "class='custom-control-label' $1";
        $new_content = preg_replace($patterns, $replacements, $new_content);

    }

    return '<li id="' . $field_id . '" class="' . $css_class . '">' . $new_content . '</li>';
}