I have a php website with iframe that resides in a different domain. I would like a script to detect if user has logged-in on the site, if not, hide iframe, else, display iframe. Is this possible at all to work on all browsers? Any suggestions from the expert?

Thank you!

Not javascript, no, as the "same domain" restriction will prevent you from accessing cookies and using javascript to access the other site.

An idea could be to store a value in a DB that is shared by the 2 sites.