Solr throws a "java.lang.IllegalArgumentException: Map size must not be negative exception" when using the PHP Serialized response writer with sharded queries.
To reproduce the issue start your preferred example and try the following query:
It is caused by the JSONWriter implementation of writeSolrDocumentList and writeSolrDocument. Overriding this two methods in the PHPSerializedResponseWriter to handle the SolrDocument size seems to solve the issue.
Attached my patch made against trunk rev 1055588.