Hey. To put things simple. geshi.php is broken on line 1947. There's a newer version out there, so I suggest you upgrade, since it's probably fixed in the newer version.
This can also be fixed by adding a simple if-statement like this:
if (isset($_SERVER['HTTP_' . GESHI_NUMBER_BIN_PREFIX_0B])) {
$geshi_number_prefix = $_SERVER['HTTP_' . GESHI_NUMBER_BIN_PREFIX_0B];
if (isset($geshi_number_prefix)) {
assert($geshi_number_prefix);
$this->language_data['NUMBERS'] = $geshi_number_prefix;
}
}