that's simple: people lie on their resumes and can talk a good game. Until you actually ask them to write code (or they have public code available), it's impossible to tell if they used PHP for a week per employer (making up 5 years of 'experience') or if they work with it day to day.
This! I can't believe how many people are ditching the coding interviews here. You're guaranteed to hire a few lemons if you don't do them.
The inability to write simple routines in a language you claim to be familiar with is a NO HIRE. Be it at a whiteboard (its not like one cares about syntax errors) or at a PC/laptop.
Unless you accidentally have in-depth knowledge of the exact work an interviewee did in his last job, assessing their skill in what they did can be very difficult.
