Ok, found it. There is a missing } in your instructions:
## Change This:
if ($Reged == "y") {
$Userlink = "<a href="{$config['phpurl']}/showprofile.php?Cat=$Cat&User=$posterid&what=search&Forum=$forumlist&Words=$URLWords&Searchpage=$Searchpage&Limit=$Limit&where=$where&Name=$Name&daterange=$daterange&newerval=$newerval&newertype=$newertype&olderval=$olderval&oldertype=$oldertype&bodyprev=$bodyprev">$PUsername</a>";
}
## to this:
if ($Reged == "y" && $displaytype == "last") {
$Userlink = "<a href="{$config['phpurl']}/showprofile.php?Cat=$Cat&User=$LastPosterId&what=search&Forum=$forumlist&Words=$URLWords&Searchpage=$Searchpage&Limit=$Limit&where=$where&Name=$Name&daterange=$daterange&newerval=$newerval&newertype=$newertype&olderval=$olderval&oldertype=$oldertype&bodyprev=$bodyprev">$PUsername</a>";
}
elseif ($Reged == "y") {
$Userlink = "<a href="{$config['phpurl']}/showprofile.php?Cat=$Cat&User=$posterid&what=search&Forum=$forumlist&Words=$URLWords&Searchpage=$Searchpage&Limit=$Limit&where=$where&Name=$Name&daterange=$daterange&newerval=$newerval&newertype=$newertype&olderval=$olderval&oldertype=$oldertype&bodyprev=$bodyprev">$PUsername</a>";
}