My company needs to hire a PHP developer, but nobody has PHP knowledge in my company and we find difficult to test for PHP skills. If it were a C/Java developer I would ask him to write a quick implementation of the Game of Life, but PHP is a completely different language.
I saw this test with interest:
http://vladalexa.com/scripts/php/test/test_php_skill.html
Anyone else has more suggestions?
Code
If you do ask the candidate to write code make sure that:
If you do ask the candidate to read code make sure that:
You can use three or more different pieces of code, start from the simpler one and only advance to the next if you see that the candidate copes with ease. Throw in some recursion, to spice things up.
Resources
Ask for a detailed list of PHP resources the candidate uses. Books, blogs, forums, magazines, etc. That's how my current employers found out about StackOverflow.
If the candidate mentions StackOverflow or Programmers, you should NOT ask or try to find out their username. If they wanted to advertise their reputation they would have included a Careers 2.0 link on their resume.
Frameworks
Every PHP developer should know of the most popular PHP frameworks:
and be fluent in at least one of them. You can have a few code samples ready for each one and ask the candidate to read and explain them, after they tell you which one they are more familiar with.
Debugging & Profiling
I've always felt that PHP developers are lacking debugging and profiling skills (perhaps only the PHP developers I've worked with). If during the discussion you find out that the candidate actively uses xdebug, don't bother with the rest of the interview and just hire them. ;)
Input sanitization
This is important. You can start with a discussion on why it's important and then ask for the most common methods to achieve it. This discussion will help you on what to ask.
Some hints:
PHP snafus
You can find a lot of PHP snafus in this excellent discussion. If you are interviewing for a senior position you should definetaly ask on some of those. Some examples:
PHP's handling of numeric values in strings:
"01a4" != "001a4" // true
"01e4" == "001e4" // also true
System.out.print("hello");
In PHP, a string is as good as a function pointer:
$x = "foo";
function foo(){ echo "wtf"; }
$x(); # "wtf"
Unit testing
Need I say more?
Conclusion
A good PHP developer should combine a variety of skills & talents:
The list goes on and on. Make sure you tailor the interview to the specific needs of the job opening, you don't want to hire just a good developer but a good developer that's great at what you immediately need him / her to do.
The test you linked to is interesting, and Yannis Rizos's answer is great, but I think what is also important is this:
If you need a good developer do not look for PHP developer. Look for a good developer who knows PHP. That means, at least half of the interview you should be asking questions that do not have much to do with PHP syntax or PHP functions.
Ask him what is MVC, what is AJAX, how HTTP works, how REST works, how SQL joins work, some data structures, some performance basics (how you measure, how you improve), testing basics, security basics (XSS, XSRF, injections, how you defend), basic programming craft knowledge as applied to typical PHP domains - web, networking, data transformations, etc.
If he's good on that, proceed to asking him to write some simple code, pick some easy task that should not take long - like finding words with most vowels in a row in the text, or reversing each word in the text, or multiplying two matrices. You can also ask him to implement the Game of Life in PHP, if you like :)
If he passes that, then you can proceed to more tricky PHP questions, but don't put too much weight into this. He can read most of it in the manual, and what he can't read, he'll learn quite quickly if he's good. PHP is built to be simple, so if he's a good programmer, and has some working knowledge of PHP, he'll probably be able to catch up. If he knows how to program in general, then learning how to program in PHP is easier than the reverse - knowing PHP syntax minutia is not important if you don't understand what unit tests are for or how to get rid of XSS problems.
While Yannis Rizos's answer is a good one - I know I wouldn't pass that test and I might use that answer as a resource to determine where to focus to improve my PHP chops, I think it would be worth your while to find a programming buddy who knows PHP to sit in on the interview process. All the questions in the previous answer are good, but without some domain knowledge it's going to be hard to assess the quality of the answers.
That said, depending on what you need as others have said general programming facility and team fit are probably more important than language-specific knowledge.
