Optional parameters are not allowed anymore before required parameters.
The tid-parameter in taxonomy_service_select_nodes($tid = '', $pager, $limit, $order) was in fact not optional before as it was not optional in the corresponding service "selectNodes":
'selectNodes' => array(
'file' => array('type' => 'inc', 'module' => 'services', 'name' => 'resources/taxonomy_resource'),
'help' => t('Returns all nodes with provided taxonomy id.'),
'access arguments' => array('access content'),
'callback' => 'taxonomy_service_select_nodes',
'args' => array(
array(
'name' => 'tid',
'type' => 'string',
'description' => t('The vocabulary ids to retrieve, separated by comma.'),
'source' => array('data' => 'tid'),
'optional' => FALSE,
),
(...)
),Closed: duplicate
3.28
Code
Thank you for the patch! Luckily this has been resolved already here #3277674: PHP8: deprecated code → .
It's available in the development snapshot, and will be available in with the next tag.