Importance of order of authors in published paper when applying for PhD positions
It depends. Authorship order doesn't always indicate who did the most work or the most important work on a paper in many fields. Some fields are alphabetical. Some fields have the lab leader or the person who won the gran last. Hopefully the admission committee understands the norms of their own field and the field you are coming from (if they are different) to be able to make a good evaluation. 

I've never served on a PhD admissions committee, but I understand that they generally will try to determine your contribution to your publications when evaluating your application.