A few tips for diagnosing problems with your LAMP application.
IT WON’T INSTALL or IT WON’T RUN AFTER INSTALLATION
Confirm that you meet the application’s minimum requirements. A common problem is incompatible PHP settings; check them with phpinfo.php.
DAMN, IT WAS WORKING BEFORE
Ask yourself what has changed on the system since it last worked. A security update to part of the LAMP stack, perhaps? Is a needed partition running out of space?
Review relevant logs. Apache’s logs are in /var/log/httpd: access_log (for successful requests) and error_log (for failed requests). I find it useful to give the command tail -f /var/log/httpd/error_log and then attempt to run the application. Error messages will scroll by in real time until stopped with Ctrl-C.
PHP has adjustable logging settings. I prefer to have the system-wide configuration /etc/php.ini set with secure, production values, even on a development box. Then I have another php.ini with development values (including verbose logging), on hand but not used. When debugging a PHP application, I copy the development php.ini to the root directory of the application, where its values will locally override the system-wide configuration.