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However the main configuration is achieved via a set of global settings stored in a global array called $GLOBALS['TYPO3_CONF_VARS'].

This chapter describes this global configuration in more details and hints at other configuration possibilities.

File LocalConfiguration.php

The global configuration is stored in file typo3conf/LocalConfiguration.php. This file overrides default settings from typo3/sysext/core/Configuration/DefaultConfiguration.php.


Since configuration settings can be manipulated from within the TYPO3 CMS backend, the typo3conf/LocalConfiguration.php must be writable by the web server user.

The local configuration file is basically a long array which is simply returned when the file is included. It represents the global TYPO3 CMS configuration. This configuration can be modified/extended/overridden by extensions, by setting configuration options inside an extension's ext_localconf.php file. See extension files and locations for more details about extension structure.

A typical content of typo3conf/LocalConfiguration.php looks like this:

return [
   'BE' => [
      'debug' => true,
      'explicitADmode' => 'explicitAllow',
      'installToolPassword' => '$P$Cbp90UttdtIKELNrDGjy4tDxh3uu9D/',
      'loginSecurityLevel' => 'normal',
   'DB' => [
      'Connections' => [
         'Default' => [
            'charset' => 'utf8',
            'dbname' => 'empty_typo3',
            'driver' => 'mysqli',
            'host' => '',
            'password' => 'foo',
            'port' => 3306,
            'user' => 'bar',
   'EXTCONF' => [
       'lang' => [
           'availableLanguages' => [
   'EXTENSIONS' => [
       'backend' => [
           'backendFavicon' => '',
           'backendLogo' => '',
           'loginBackgroundImage' => '',
           'loginFootnote' => '',
           'loginHighlightColor' => '',
           'loginLogo' => '',
       'extensionmanager' => [
           'automaticInstallation' => '1',
           'offlineMode' => '0',
       'scheduler' => [
           'maxLifetime' => '1440',
           'showSampleTasks' => '1',
   'FE' => [
      'debug' => true,
      'loginSecurityLevel' => 'normal',
   'GFX' => [
      'jpg_quality' => '80',
   'MAIL' => [
      'transport_sendmail_command' => '/usr/sbin/sendmail -t -i ',
   'SYS' => [
      'devIPmask' => '*',
      'displayErrors' => 1,
      'encryptionKey' => '0396e1b6b53bf48b0bfed9e97a62744158452dfb9b9909fe32d4b7a709816c9b4e94dcd69c011f989d322cb22309f2f2',
      'exceptionalErrors' => 28674,
      'sitename' => 'New TYPO3 site',

As you can see, the array is structured on two main levels. The first level corresponds roughly to a category, the second one being properties, which may themselves be arrays.

The configuration categories are:


Options related to the TYPO3 CMS backend.


Database connection configuration.


Extension installation options.


Backend related language pack configuration resides here.


Extension configuration.


Frontend-related options.


Settings for tuning HTTP requests made by TYPO3.


Options related to image manipulation.


Options related to the sending of emails (transport, server, etc.).


General options which may affect both the frontend and the backend.

Details on the various configuration options can be found in the Install Tool as well as the TYPO3 source at typo3/sysext/core/Configuration/DefaultConfigurationDescription.yaml. The documentation shown in the Install Tool is automatically extracted from those values of DefaultConfigurationDescription.yaml.

The Install Tool provides various dedicated modules that change parts of LocalConfiguration.php, those can be found in ADMIN TOOLS > Settings, most importantly section Configure installation-wide options:

Configure installation-wide options in Install Tool with an active search

File AdditionalConfiguration.php

Although you can manually edit the typo3conf/LocalConfiguration.php file, it is limited in scope because the file is expected to return a PHP array. Also the file is rewritten every time an option is changed in the Install Tool or some other operation (like changing an extension configuration in the Extension Manager). Thus custom code cannot reside in that file.

Such code should be placed in the typo3conf/AdditionalConfiguration.php file. This file is never touched by TYPO3, so any code will be left alone.

Furthermore this file is loaded after typo3conf/LocalConfiguration.php, which means it represents an opportunity to change global configuration values programmatically if needed.

typo3conf/AdditionalConfiguration.php is a plain PHP file. There are no specific rules about what it may contain. However since the code it contains is included on every request to TYPO3 CMS - whether frontend or backend - you should avoid inserting code which requires heavy duty processing.

Changing the database hostname for development machines
if (getenv('TYPO3_CONTEXT') == 'Development') {
    $GLOBALS['TYPO3_CONF_VARS']['DB']['Connections']['Default']['host'] = 'mysql-be';

File DefaultConfiguration.php

TYPO3 CMS comes with some default settings, which are defined in file typo3/sysext/core/Configuration/DefaultConfiguration.php.

This is the base configuration, the other files like LocalConfiguration.php just overlay it.

Here is an extract of that file:

return [
        'GFX' => [
                'thumbnails' => true,
                'thumbnails_png' => true,
                'gif_compress' => true,
                'imagefile_ext' => 'gif,jpg,jpeg,tif,tiff,bmp,pcx,tga,png,pdf,ai,svg',
                // ...
        // ...

You will probably find it interesting to take a look at that file, which also contains values not displayed in the Install Tool and thus not easily available for modification.