In Use of read preferences with commands, the Server Selection spec stats:

The generic command method MUST act as a read operation for the purposes of server selection.

The generic command method has a default read preference of mode 'primary'. The generic command method MUST ignore any default read preference from client, database or collection configuration. The generic command method SHOULD allow an optional read preference argument.

php_phongo_manager_select_server() conflict with this, as it inherits the client's read preference if none is provided.