code: public function findFile($class)
      {
          // class map lookup
          if (isset($this->classMap[$class])) {
              return $this->classMap[$class];
          }
          if ($this->classMapAuthoritative || isset($this->missingClasses[$class])) {
              return false;
          }
          if (null !== $this->apcuPrefix) {
              $file = apcu_fetch($this->apcuPrefix.$class, $hit);
              if ($hit) {
                  return $file;
              }
          }
  
          $file = $this->findFileWithExtension($class, '.php');
  
          // Search for Hack files if we are running on HHVM
          if (false === $file && defined('HHVM_VERSION')) {
              $file = $this->findFileWithExtension($class, '.hh');
          }
  
          if (null !== $this->apcuPrefix) {
              apcu_add($this->apcuPrefix.$class, $file);
          }
  
          if (false === $file) {
              // Remember that this class does not exist.
              $this->missingClasses[$class] = true;
          }
  
          return $file;
      }
docs: Finds the path to the file where the class is defined.
  
  @param string $class The name of the class
  
  @return string|false The path if found, false otherwise