You don't need to use tabs anymore in PhpWiki. Consider this page obsolete. See TextFormattingRules for how to use PhpWiki markup.
VariousBrowsers? have trouble with the tab character used in the TextFormattingRules. If you can't type a tab, or, if you are fixing up a page written by someone who didn't type tabs, then you should have us convert spans of spaces to tabs for you. You request space to tab conversion with the checkbox that appears near the bottom of EditText and EditCopy? pages. It looks something like this...
[o] I can't type tabs. Please ConvertSpacesToTabs for me when I save.
A span of spaces must be at least three spaces long to be recognized as a tab. Use multiples of eight spaces to make multiple tabs. Each span of eight spaces will convert to one tab. If the last span comes up a little short, that's ok, as long as there are at least three spaces in the last (only) span.
Check your work. This sort of heuristic can lead to unexpected results.
[Maintenance note: This page is seen when the user clicks on the "ConvertSpacesToTabs" link on any edit screen. It describes what happens when you ask the system to do this, why you would want it to, and ways to avoid the spaces-instead-of-tabs problem.]