WordPressのカスタマイズ画面でのフォーム項目の出し分け active_callback

add_action( 'customize_register', 'lightning_customize_register_basic' );
function lightning_customize_register_basic( $wp_customize ) {

    $wp_customize->add_section(
        'lightning_basic',
        array(
            'title'    => __( 'Basic settings', 'lightning' ),
        )
    );
    $wp_customize->add_setting( 'demo_radio_control', array(
        'default'        => 'a',
     ) );

     $wp_customize->add_control( 'demo_radio_control', array(
        'label'      => 'radio_control',
        'section'    => 'lightning_basic',
        'settings'   => 'demo_radio_control',
        'type'       => 'radio',
        'choices'    => array(
        'a' => 'Choice A',
        'b' => 'Choice B',
        ),
     ) );

     $wp_customize->add_setting( 'choice_a_text', array(
        'default' => '',
     ) );

     $wp_customize->add_control( 'choice_a_text', array(
        'label'      => 'Choice A: ',
        'section'    => 'lightning_basic',
        'type'       => 'text',
        'active_callback' => 'choice_a_callback',
     ) );
     function choice_a_callback( $control ) {
        if ( $control->manager->get_setting('demo_radio_control')->value() == 'a' ) {
           return true;
        } else {
           return false;
        }
     }
}