Annoying problem with apache22 / php5 - how to investigate?

[Per olof Ljungmark]

Bruce Ferrell wrote:
> 
> Per olof Ljungmark wrote:
>> Hi,
>>
>> We run 7-STABLE and apache22 with php5 serving pages from a webmail app
>> (Horde).
>>
>> Randomly (as it seems at least), there is a 500 (Internal server error)
>> and a blank page is presented to the user like
>>
>> [04/Jul/2009:15:19:37 +0200] "GET
>> /services/portal/sidebar.php?httpclient=1 HTTP/1.1" 500 -
>>
>> There are no other messages in the logs, not even with LogLevel debug.
>> The problem has survived several both OS and port upgrades and I really
>> need to track this down now.
>>
>> Question: What OS tools would be the best to further analyze this?
>> Someone with more exparience running this combo perhaps would know?
>>
>> Thanks a lot!
>>
>> -- 
>> per
> 
> I'd look in the Apache error_log
> 

I've looked, as stated above. Even with LogLevel debug there is not a 
trace. Likewise, I have E_ALL set in php.ini.

So, I need to dig deeper inte the workings of Apache with the aid of the 
right tools.