check posted data for array and string

$posted_field_name   = 'mnm_quantity';

$child_item_quantity = ! empty( $_REQUEST[ $posted_field_name ] ) && ! empty( $_REQUEST[ $posted_field_name ][ $child_product_id ] ) ? intval( $_REQUEST[ $posted_field_name ][ $child_product_id ] ) : 0;

// One more check for inputs that are not arrays. Ajax add to cart will frequently not handle array inputs well (esp in themes) so as a last resort let's check for a single string key: ie: "mnm_quantity[99]".
if ( $child_item_quantity <= 0 ) {
    $posted_field_name   .= '[' . $child_product_id . ']';
    $child_item_quantity = ! empty( $_REQUEST[ $posted_field_name ] ) ? intval( $_REQUEST[ $posted_field_name ] ) : 0;
}