[eluser]Aken[/eluser]
I'd suggest using site_url() if you're dealing with URLs and links to pages on your website. base_url() is more for static assets.
I'd also put the suffixed content into the function's parameter, rather than outside. That'll guarantee a proper URL, just in case you toss in a slash in the wrong place or something by mistake.
Code:
function admin_url($url = '')
{
return site_url('admin123/'.$url);
}