https://api.drupal.org/api/drupal/core%21modules%21system%21tests%21modules%21form_test%21src%21Form%21FormTestPlaceholderForm.php/9
FormTestPlaceholderForm.php | Drupal API
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https://api.drupal.org/api/drupal/core%21modules%21system%21tests%21modules%21form_test%21src%21Form%21FormTestPlaceholderForm.php/class/FormTestPlaceholderForm/10
class FormTestPlaceholderForm | Drupal API
classdrupalapi