Questions for mid-level PHP developer candidates

I often get CV's from developers applying for positions. Some colleges give people a certificate without really giving the candidate any problem solving skills or real understanding of theory. Here are some standard questions that I ask candidates to complete with pen and paper without access to Google. They cover basic OOP theory, logic, basic PHP syntax, and try to get some idea of the candidates passion for learning.

In the rare occasion that a candidate actually bothers to investigate the company and finds my blog they will naturally be expected to do well on this quiz.  I guess that's bonus marks for being prepared :p

PHP quiz
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1)  Explain what SQL injection is and give TWO ways to combat it
2)  If you type hint an interface name in a function argument what sort of variables can you pass?

3)  What is an abstract class?
4)  How would you call the construct method of a parent class inside 

a child of that class?
5)  Given two variables $a and $b which contain integer numbers.  

Swap the values of $a and $b without declaring a third variable and using 

only the mathematical functions +,*,-,/
6)  Define a class called House that has an owner property and a method 

called sell that accepts a string parameter which changes the owner
7)  Explain call by value and call by reference.  Which method does PHP5 

use when passing primitive variable types and objects?
8)  What does AJAX stand for?  Write a jQuery AJAX call to 'weather.php' 

which updates the contents of with the results from that file
9)  What is the safest PHP function to use to filter output to prevent XSS?
10) What is the difference between GET and POST?
11) What is your approach to unit and integration testing?

12) What are traits used for and how would you include one in your class?
13) What design patterns are you familiar with?  What do you think about 

the use of the Singleton pattern in PHP?
14) Write a program to roll two six-sided dice 10,000 times.  Sum the two

values on each roll.  At the end of the program run output the average sum 

of all the rolls.
15) What is the value of $a if $a = ( '42' === 42 ) ? 'answer one' : 'answer two';

Feel free to use any or all of these questions if you like them.  They're awkwardly formatted on the blog because of the template I'm using.  I have shared a raw copy on Google Docs.

I have seen question number 5 done in a single line by the way (usually it takes three).