Returns the position of the file pointer referenced by stream.
The file pointer must be valid, and must point to a file successfully opened by fopen or popen. ftell gives undefined results for append-only streams (opened with "a" flag).
Returns the position of the file pointer referenced by stream as an integer; i.e., its offset into the file stream.
If an error occurs, returns false.
Because PHP's integer type is signed and many platforms use 32bit integers, some filesystem functions may return unexpected results for files which are larger than 2GB.