For Joomla 1.5 --

to fix the undefined index uri.php line 161 issue at installation:

Source - Joomla.org

line 161, change strlen to isset
 
if (strlen($_SERVER['QUERY_STRING']) && strpos($_SERVER['REQUEST_URI'], $_SERVER['QUERY_STRING']) === false) {
$theURI .= '?'.$_SERVER['QUERY_STRING'];
}