# Constants¶

Constants in TYPO3 define paths and database information. These values are global and cannot be changed. Constants are defined at various points during the bootstrap sequence.

To make the information below a bit more compact, namespaces were left out. Here are the fully qualified class names referred to below:

• “SystemEnvironmentBuilder” = \TYPO3\CMS\Core\Core\SystemEnvironmentBuilder
• “Bootstrap” = \TYPO3\CMS\Core\Core\Bootstrap

Check SystemEnvironmentBuilder::defineBaseConstants() for more constants.

## Paths¶

### TYPO3_mainDir¶

This is the directory of the backend administration for the sites of this TYPO3 installation. Hardcoded to typo3/. Must be a subdirectory to the website.

Defined in:
SystemEnvironmentBuilder::defineBaseConstants()
Available in Frontend:
Yes

## Filetypes¶

Different types of files constants are defined in TYPO3\CMS\Core\Resource\AbstractFile. These constants are available for different groups of files as documented in https://www.iana.org/assignments/media-types/media-types.xhtml

These file types are assigned to all FAL resources. They can, for example, be used in Fluid to decide how to render different types of files.

Constant Value Description
FILETYPE_UNKNOWN 0 Unknown
FILETYPE_TEXT 1 Any kind of text
FILETYPE_IMAGE 2 Any kind of image
FILETYPE_AUDIO 3 Any kind of audio
FILETYPE_VIDEO 4 Any kind of video
FILETYPE_APPLICATION 5 Any kind of application

## HTTP status codes¶

The different status codes available are defined in TYPO3\CMS\Core\Utility\HttpUtility. These constants are defined as documented in https://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml