Both functions (either as method of class user of or as method of class acp_users) in question are defined this way (simplified):

function optionget( ... $data = false)

{

	...

	$var = ($data) ? $data : $this->data['user_options'];

	...

}

The problem in here is that $data will also evaluate to FALSE if it was handed over nonetheless. Consider a case where I use the $data parameter by giving the value 0 (ordinal zero) or '' (empty string) - this value wouldn't be used, instead

$this->data['user_options']

will be used. Which is fatal.

A correct approach would be to test without invoking typecasts:

function optionget( ... $data = false)

{

	...

	$var = ($data !== false) ? $data : $this->data['user_options'];

	...

}

I submitted this as minor bug because right now no code in phpBB uses the $data parameter, so nothing is affected. Only exception: the STK uses it in /tools/admin/reparse_bbcode.php, but obviously nobody ever had problems with it.