Sets the file position indicator for the file referenced
by handle.The new position,
measured in bytes from the beginning of the file, is obtained
by adding offset to the
position specified by
whence, whose values are defined as follows:
If whence is not
specified, it is assumed to be
SEEK_SET.
Upon success, returns 0; otherwise, returns -1. Note
that seeking past EOF is not considered an error.
May not be used on file pointers returned by fopen() if
they use the "http://" or "ftp://" formats.
注: The whence argument was added after PHP 4.0.0.