WordPress - add_filter函数和字符串参数？

I found this filter in a piece of code:

$output  = apply_filters( 'envira_gallery_output_dynamic_position', $output, $id, $item, $data, $i, 'bottom-right' );

My assumption was, that I can make use of it like this:

add_filter( 'envira_gallery_output_dynamic_position', 'custom_gallery_output_dynamic_position', 10, 6);

function custom_gallery_output_dynamic_position($output, $id, $item, $data, $i, 'bottom-right' ) {

But because of the 'bottom-right' I get a 500 server error:

PHP Parse error:  syntax error, unexpected ''bottom-right'' (T_CONSTANT_ENCAPSED_STRING), expecting variable (T_VARIABLE)

Does anybody know how I can use this filter correctly?

Thanks!

There must be variable name:

function custom_gallery_output_dynamic_position($output, $id, $item, $data, $i, $position) {

You can define it with default value:

function custom_gallery_output_dynamic_position($output, $id, $item, $data, $i, $position='bottom-right') {