Let's take a look at what debugging possibilities TYPO3 provides.

Debugging PHP code

The TYPO3 Core provides a simple debug() (defined in EXT:core/Classes/Core/GlobalDebugFunctions.php). It wraps around \TYPO3\CMS\Core\Utility\DebugUtility::debug() and will output debug information only if it matches a set of IP addresses (defined in $GLOBALS['TYPO3_CONF_VARS']['SYS']['devIPmask']).

For example, the following code:

Extension examples, file Classes/Controller/ModuleController.php
use TYPO3\CMS\Core\Utility\DebugUtility;

class ModuleController extends ActionController implements LoggerAwareInterface
    protected function debugCookies() {
        DebugUtility::debug($_COOKIE, 'cookie');

will produce such an output:

Typical TYPO3 debug output

In general, look at class \TYPO3\CMS\Core\Utility\DebugUtility for useful debugging tools.

Backend debug mode

To display additional debug information in the backend, set $GLOBALS['TYPO3_CONF_VARS']['BE']['debug'] in config/system/settings.php and log in with an administrator account.

It shows for example the names of fields and in case of select, radio and checkbox fields the values in addition, which are generated by the FormEngine. These can be used to set access permissions or configuration using TSconfig.

If EXT:lowlevel is installed, the name of the database table or field is appended to the select options in the System > DB Check > Full Search module.

Additionally, in debug mode, the page renderer does not compress or concatenate JavaScript or CSS resources.