6 files declare their use of PoHeader

PoDatabaseReader.php in drupal/core/modules/locale/lib/Drupal/locale/PoDatabaseReader.php
Definition of Drupal\locale\PoDatabaseReader.
PoDatabaseWriter.php in drupal/core/modules/locale/lib/Drupal/locale/PoDatabaseWriter.php
Definition of Drupal\locale\PoDatabaseWriter.
PoMemoryWriter.php in drupal/core/lib/Drupal/Component/Gettext/PoMemoryWriter.php
Definition of Drupal\Component\Gettext\PoMemoryWriter.
PoMetadataInterface.php in drupal/core/lib/Drupal/Component/Gettext/PoMetadataInterface.php
Definition of Drupal\Component\Gettext\PoMetadataInterface.
PoStreamReader.php in drupal/core/lib/Drupal/Component/Gettext/PoStreamReader.php
Definition of Drupal\Component\Gettext\PoStreamReader.
PoStreamWriter.php in drupal/core/lib/Drupal/Component/Gettext/PoStreamWriter.php
Definition of Drupal\Component\Gettext\PoStreamWriter.