Two words – variable scope. Use a closure instead and pass $rstax
through with use
:
add_filter(
'new_royalslider_posts_slider_query_args',
function ( $args ) use ( $rstax ) {
return array(
'post_type' => 'carousel-slide',
'orderby' => array(
'menu_order' => 'ASC'
),
'tax_query' => array(
array(
'taxonomy' => 'carousel-slide-category',
'field' => 'slug',
'terms' => $rstax,
),
),
)
}
);