Question

Explain this interview question to me:

Q: If the variable $a is equal to 5 and variable $b is equal to character a, what’s the value of $$b?

A: 5, it’s a reference to existing variable.

Answer 1

That’s a variable variable. PHP will look up the variable with the name stored in the string $b. So if $b == 'a' then $$b == $a.

It’s a lot like pointers in C, except they use variable name strings instead of memory addresses to point to each other. And you can dereference as many times as you want:

$a = 5;
foreach (range('b', 'z') as $L) {
  $$L = chr(ord($L) - 1);
}
echo $$$$$$$$$$$$$$$$$$$$$$$$$$z;

Output:

5