public function getETag()
    {
        return '"'.sha1(
            fileinode($this->path).
            filesize($this->path).
            filemtime($this->path)
        ).'"';
    }
public function getETag()
    {
        return '"'.sha1(
            fileinode($this->path).
            filesize($this->path).
            filemtime($this->path)
        ).'"';
    }
public function getETag ( ) { return '"' . sha1 ( fileinode ( $ this -> path ) . filesize ( $ this -> path ) . filemtime ( $ this -> path ) ) . '"' ; }
Returns the ETag for a file.

An ETag is a unique identifier representing the current version of the file. If the file changes, the ETag MUST change.
The ETag is an arbitrary string, but MUST be surrounded by double-quotes.

Return null if the ETag can not effectively be determined

@return mixed
Returns the ETag for a file .