Joshua Thjissen has revisited some of his PHP internal function statistics, an update from this previous post with some results showing the most (and least) used internal PHP functions in several large projects from GitHub.
A lot of people are asking about functions like isset, empty, print, echo etc, as they are not present in the current result list. The thing is, is that these are not really functions, but language constructs. This means that PHP treats them a bit different than normal functions, and this results sometimes in seemingly "strange" behaviour when trying to use them like regular functions.
He's updated his results, though, to reflect the usage of these "functions" and shared the numbers. Not surprisingly, these constructs show up pretty highly in the new "top 22" list he's produced. With the inclusion of the constructs, the number one item on the list is now "isset" by a very large margin. The full results can be found in this gist.