add_action( 'rank_math/vars/register_extra_replacements', function() {
    rank_math_register_var_replacement(
        'woo_full_desc',
        [
            'name'        => esc_html__( 'Woo Full Desc.', 'rank-math' ),
            'description' => esc_html__( 'Woo Full Description...', 'rank-math' ),
            'variable'    => 'woo_full_desc',
            'example'     => woo_full_desc_callback(),
        ],
        'woo_full_desc_callback'
    );
} );
function woo_full_desc_callback() {
    global $post;

    if ( empty( $post ) ) {
        return 'Product Description';
    }

    $product_description = substr(wp_strip_all_tags( $post->post_content ), 0, 169) . '...';

    return $product_description;
}