Send Array as an parameter of Function

I have a function that takes array as a parameter.
function name_function($Usersarray){
do_something;
}
$server->register(
‘name_function’,
array(‘Usersarray’=>‘tns:array’),
array(‘return’=>‘xsd:string’),
API_NAMESPACE,
false, false, false
);

I am getting this error:
contains an invalid part (‘Usersarray’): invalid xsd type or elements.

I also tried with: array(‘Usersarray’=>‘tns:Array’) and array(‘Usersarray’=>‘tns:Userarray’) but without success.

Also, when I change tns:‘array’ to xsd:‘string’, it’s working without any problem.

Any help?