If you have already documented your code elements via docblocks, then when you use those elements in your code, the hover popups feature of Eclipse will indeed show you what's in their docblocks. This doesn't require you to have the other file open, e.g. writing code in test.php that uses class Foo from foo.php... you're not required to have foo.php open in the editor in order for your Foo object in test.php to have hover popup docblock info visible.
If such hover popups are not showing docblock info to you, then you might need to revisit your buildpath/includepath settings in your PHP project, particularly if the elements you're trying to use are actually defined in another project than the one you're working in.