Check the following requests, the link attribute is missing from questions objects in responses. I checked with the default filter and also with custom filters (both safe and unsafe).
https://api.stackexchange.com/2.0/tags/php/faq?site=stackoverflow http://api.stackexchange.com/2.0/tags/php/faq?site=stackoverflow&filter=!.bzEk)B2 https://api.stackexchange.com/2.0/tags/php/faq?site=stackoverflow&filter=DLsN_e.SW