iPhone 4 screen lock through unsolicited ad On my iPhone 4 A1332 I had a pop-up this morning from daily-offers-online.com
which I could not remove.

I had no access to Safari until I clicked on the pop-up's OK button (which I did by accident).

How would I know if there is (by having done this) any bug/virus installed on my iPhone? …and how could it be removed?
It's a JavaScript alert from the page that you're currently viewing. It's not malware or anything installed on the device—just JavaScript.

Tap to dismiss the dialog then leave the advertiser's webpage. It doesn't lock the screen, just presents a modal dialog in front of the content.