You cannot, this information is simply not available through any standard interface (except for the window to which you already have a link). You can understand why. You would not want the site owner on one of your tabs to be able to find out what all your other tabs showed, this will be a massive invasion of your privacy.
For windows that you already have a link to, you can get the URL of what this window shows ( window.location.href ), and possibly its parent window ( window.parent.location.href ) ( window.top.location.href ), and sub-frames ( window.frames[n].location.href - I think). But this will not give you the tabs that you requested.
This information is most likely available through the extension mechanism of various browsers (Firefox add-ons, Chrome extensions, etc.), but it will only be a browser extension that requires explicit installation by the user, and (currently) requires recording one for of every browser provider where not all providers offer an extension mechanism.
Separately: this information, of course, is not sent to the server (you noted your php question).
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