core-bundle/src/Resources/contao/library/Contao/Files.php
public function fopen($strFile, $strMode)
	{
		$this->validate($strFile);

		return fopen($this->strRootDir . '/' . $strFile, $strMode);
	}
public function fopen($strFile, $strMode)
	{
		$this->validate($strFile);

		return fopen($this->strRootDir . '/' . $strFile, $strMode);
	}
Open a file and return the handle

@param string $strFile The file name
@param string $strMode The operation mode

@return resource The file handle