What's the proper way to sanitize checkbox value sent to the database

I have use this function it working.

/************************************************************************
************** How to sanitize checkbox*************************
************************************************************************/


function theme_slug_customizer( $wp_customize ) {           

    //your section
        $wp_customize->add_section( 
            'theme_slug_customizer_your_section', 
            array(
                'title' => esc_html__( 'Your Section', 'theme_slug' ),
                'priority' => 150
            )
        );      


    //checkbox sanitization function
        function theme_slug_sanitize_checkbox( $input ){

            //returns true if checkbox is checked
            return ( isset( $input ) ? true : false );
        }


    //add setting to your section
        $wp_customize->add_setting( 
            'theme_slug_customizer_checkbox', 
            array(
                'default' => '',
                'sanitize_callback' => 'theme_slug_sanitize_checkbox'
            )
        );

        $wp_customize->add_control( 
            'theme_slug_customizer_checkbox', 
            array(
                'label' => esc_html__( 'Your Setting with Checkbox', 'theme_slug' ),
                'section' => 'theme_slug_customizer_your_section',
                'type' => 'checkbox'
            )
        );      

}
add_action( 'customize_register', 'theme_slug_customizer' );










/************************************************************************
************** How to sanitize radio box *************************
************************************************************************/


function theme_slug_customizer( $wp_customize ) {           

    //your section
        $wp_customize->add_section( 
            'theme_slug_customizer_your_section', 
            array(
                'title' => esc_html__( 'Your Section', 'theme_slug' ),
                'priority' => 150
            )
        );      


    //radio box sanitization function
        function theme_slug_sanitize_radio( $input, $setting ){

            //input must be a slug: lowercase alphanumeric characters, dashes and underscores are allowed only
            $input = sanitize_key($input);

            //get the list of possible radio box options 
            $choices = $setting->manager->get_control( $setting->id )->choices;

            //return input if valid or return default option
            return ( array_key_exists( $input, $choices ) ? $input : $setting->default );                

        }


    //add setting to your section
        $wp_customize->add_setting( 
            'theme_slug_customizer_radio', 
            array(
                'sanitize_callback' => 'theme_slug_sanitize_radio'
            )
        );

        $wp_customize->add_control( 
            'theme_slug_customizer_radio', 
            array(
                'label' => esc_html__( 'Your Setting with Radio Box', 'theme_slug' ),
                'section' => 'theme_slug_customizer_your_section',
                'type' => 'radio',
                'choices' => array(
                    'one' => esc_html__('Choice One','theme_slug'),
                    'two' => esc_html__('Choice Two','theme_slug'),
                    'three' => esc_html__('Choice Three','theme_slug')               
                )
            )
        );      

}
add_action( 'customize_register', 'theme_slug_customizer' );

I would use the filter_var() function. It has some predefined filters that you can use depending on what kind of data you are expecting such as string, number, etc.

So to sanitize for a number:

$sanitizedNum = filter_var($yourVar, FILTER_SANITIZE_NUMBER_INT);

For a string you would just change "_NUM_INT" to "_STRING".

Wrap those in a custom function then.

I mean that the value of checkbox or radio is often a integer value. If is a integer value, then set it to a integer as solid filter.

$checkbox = (int) $checkbox;

If you use strings on the radio items, then use esc_attr to filter solid. The function sanitize_text_field have also a filter, that other plugins can change the output, maybe not helpful for your goal. THe function is more for filter input from users or from database. esc_attrhave also a filter, but is more solid for your requirements.

More information you can find on the codex page about validation.