I'm not sure if this is the right stackexhange website, but :

I have three candidates coming in tomorrow. One has 15 years of experience in PHP, and the two others have about 1 year of experience in PHP/ frontend development.

For the last ones I was thinking about a test where they would have to develop a web app allowing users to manage other users, as in : Display a list of users, display a single user, modify an user, and add extended properties to an user. This way it would feature html, css, js, ajax, php and SQL.

Do you think this would be a good test?

What test should I give to the first one? He needs something much more difficult, I guess.

I'm also listening, if you have any advice/ideas about what makes a good developer, and what I should pay attention to in the guys' codes. I was also considering thinking outside of the box, more algorithm-related, and asked him to make the fastest function to tell if a number is a prime number, because there are a lot of optimizations you can apply to such a function.

They have one day to do it.