core-bundle/src/Resources/contao/library/Contao/File.php
public function truncate()
	{
		if (\is_resource($this->resFile))
		{
			ftruncate($this->resFile, 0);
			rewind($this->resFile);
		}

		return $this->write('');
	}
public function truncate()
	{
		if (\is_resource($this->resFile))
		{
			ftruncate($this->resFile, 0);
			rewind($this->resFile);
		}

		return $this->write('');
	}
Truncate the file and reset the file pointer

@return boolean True if the operation was successful