Why the Zend Studio doesn't recognize some functions?
Here how I have solved the issue:
Under Preferences in the Zend Studio application I have added the correct path of the interpreter. In my case: /usr/bin/php
Then I have closed and open again the Zend Studio and the issue is disappeared.
Zend Studio Preferences
It recognizes them, it's just a notice that you should use global namespace for these functions:
\is_numeric($id);
\print_r($event);
empty()
is not function, but language construct.
It's likely possible to exclude these notices somewhere in the settings.