What about back compat? How might this affect those upgrading from 4.0?

Upgrading from 4.0?

I don't see any harm in the ResponseWriters being configurable. JSONResponseWriter already is (as far as Content-Type header in concerned).
Sending text/x-php;charset=UTF-8 as default however might be an issue, yes (if anyone really relies on text/plain for PHP and PHPs).
But since the Content-Type header of the ResponseWriters would be configurable you could still overwrite the defaults in solrconfig.xml just like the example does with the JSONResponseWriter:

<queryResponseWriter name="json" class="solr.JSONResponseWriter">
	<!-- For the purposes of the tutorial, JSON responses are written as
	  plain text so that they are easy to read in *any* browser.
	  If you expect a MIME type of "application/json" just remove this override.
	-->
	<str name="content-type">text/plain; charset=UTF-8</str>
</queryResponseWriter>

Okay - if no one complains I'll just commit this change then.

Could you update this patch to 4x or trunk Dominik - looks like some thing have changed. I don't have the time to do it myself right now, but if you do, I can certainly commit it.

Will do, might have to wait until Sunday, though.

Patch to make Content-Type header configurable in PHPResponseWriters.

Had to add two patches for trunk and branch_4x separately, due to changes in trunk ("missing @Overrides").

[trunk commit] Mark Robert Miller
http://svn.apache.org/viewvc?view=revision&revision=1423121

SOLR-4110: Configurable Content-Type headers for PHPResponseWriters and PHPSerializedResponseWriter.

[branch_4x commit] Mark Robert Miller
http://svn.apache.org/viewvc?view=revision&revision=1423123

SOLR-4110: Configurable Content-Type headers for PHPResponseWriters and PHPSerializedResponseWriter.

Thanks Dominik!