YAML is used in TYPO3 for various configurations; most notable are


TYPO3 is using a custom YAML loader for handling YAML in TYPO3 based on the Symfony YAML package. It’s located at \TYPO3\CMS\Core\Configuration\Loader\YamlFileLoader and can be used when YAML parsing is required.

The TYPO3 Core YAML resolves environment variables. Resolving of variables in the loader can be enabled or disabled via flags. For example, when editing the site configuration through the backend interface the resolving of environment variables needs to be disabled to be able to add environment configuration through the interface.

The format for environment variables is %env(ENV_NAME)%. Environment variables may be used to replace complete values or parts of a value.

The YAML Loader class has two flags: PROCESS_PLACEHOLDERS and PROCESS_IMPORTS.

  • PROCESS_PLACEHOLDERS decides whether or not placeholders (%abc%) will be resolved.
  • PROCESS_IMPORTS decides whether or not imports (imports key) will be resolved.

Use the method YamlFileLoader::load() to make use of the loader in your extensions:

use TYPO3\CMS\Core\Configuration\Loader\YamlFileLoader;

// ...

YamlFileLoader::load(string $fileName, int $flags = self::PROCESS_PLACEHOLDERS | self::PROCESS_IMPORTS)

Configuration files can make use of import functionality to reference to the contents of different files.


  - { resource: "EXT:rte_ckeditor/Configuration/RTE/Processing.yaml" }
  - { resource: "misc/my_options.yaml" }
  - { resource: "../path/to/something/within/the/project-folder/generic.yaml" }

Custom Placeholder Processing

It is possible to register custom placeholder processors to allow fetching data from different sources. To do so, register a custom processor via LocalConfiguration.php:

$GLOBALS['TYPO3_CONF_VARS']['SYS']['yamlLoader']['placeholderProcessors'][\Vendor\MyExtension\PlaceholderProcessor\CustomPlaceholderProcessor::class] = [];

There are some options available to sort or disable placeholder processors if necessary:

$GLOBALS['TYPO3_CONF_VARS']['SYS']['yamlLoader']['placeholderProcessors'][\Vendor\MyExtension\PlaceholderProcessor\CustomPlaceholderProcessor::class] = [
   'before' => [
   'after' => [
   'disabled' => false,

New placeholder processors must implement the \TYPO3\CMS\Core\Configuration\Processor\Placeholder\PlaceholderProcessorInterface An implementation may look like the following:

class ExamplePlaceholderProcessor implements PlaceholderProcessorInterface
   public function canProcess(string $placeholder, array $referenceArray): bool
      return strpos($placeholder, '%example(') !== false;

   public function process(string $value, array $referenceArray)
      // do some processing
      $result = $this->getValue($value);

      // Throw this exception if the placeholder can't be substituted
      if ($result === null) {
         throw new \UnexpectedValueException('Value not found', 1581596096);
      return $result;

   private function getValue(string $value): ?string
      // implement logic to fetch specific values from an external service
      // or just add simple mapping logic - whatever is appropriate
      $aliases = [
         'foo' => 'F-O-O',
         'bar' => 'ARRRRR',
      return $aliases[$value] ?? null;

This may be used for example in the site configuration:

someVariable: '%example(somevalue)%'
anotherVariable: 'inline::%example(anotherValue)%::placeholder'

If a new processor returns a string or number, it may also be used inline as above. If it returns an array, it cannot be used inline since the whole content will be replaced with the new value.