These features are included in PHPExcel, they are static methods of the PHPEXcel_Cell class:
PHPExcel_Cell::columnIndexFromString([string $pString = 'A'])
return: column index (base 1)
PHPExcel_Cell::stringFromColumnIndex([int $pColumnIndex = 0])
The column index is 0-based.
In the documentation of the API, you can find them in the PHPExcel_Cell package, class PHPExcel_Cell.
I flew over your function at first glance, it is not false, but does not support the columns of Excel 2007 +, which limit 16384 (XFD), columns in three letters.
Note: In my sentence, "Mark" is Mark Baker, the remarkable PHPExcel Coordinator.