Finally found a solution to the problem which might help others in the future. Not sure if it's the best solution to this problem, but it worked.
Willem-Jan pointed me in the right direction about encoding. I used Firefox's add-on Live HTTP Headers to investigate the way the URL was sent to Delicious. The link "http://feeds.delicious.com/v2/json/username/tag’12?count=100" tranformed into "http://feeds.delicious.com/v2/json/username/tag%E2%80%9912?count=100".
When using the already translated URL as $url variable the request displayed the correct results.