This is more of a personal notes for myself but I am sure others will find it useful.

Apache:

/usr/local/apache/logs/error_log

All exceptions caught by httpd, along with standard error output form CGI applications are logged here.  The first place you should look when httpd crashes, or you incur errors when accessing a website.

/usr/local/apache/logs/suexec_log

Contains auditing information reported by suexec each time a CGI applicaiton is executed.  If you receive an internal server error, with no relevant information being reported to the Apache error_log, check here for potential suexec policy violations.

Domain specific access logs are located here:

/usr/local/apache/domlogs/

PHP

To find out which php.ini is used for PHP configuration (to save you frustration of editing a non active php.ini) find it with the following simple command:
php -i | grep php.ini

where -i switch is used to give you PHP information (obviously). To see other useful switches run

php -h

to locate where php is use:

which php

(but you already knew this – right ?   )

*** Some of these tips are applicable not only to CentOS and cPanel configuration.

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