code: protected function substitutePlaceholderNames(array $matches)
      {
          foreach ($this->placeholders as $placeholder => $name) {
              if (isset($matches[$placeholder])) {
                  $matches[$name] = $matches[$placeholder];
                  unset($matches[$placeholder]);
              }
          }
  
          return $matches;
      }
docs: Iterates over [[placeholders]] and checks whether each placeholder exists as a key in $matches array.
  When found - replaces this placeholder key with a appropriate name of matching parameter.
  Used in [[parseRequest()]], [[createUrl()]].
  
  @param array $matches result of `preg_match()` call
  @return array input array with replaced placeholder keys
  @see placeholders
  @since 2.0.7