PHP variable variables

Question

I know what you can do: $hash('foo')and $$foo, as well $bar[$foo], what is called each of these things?

Answer 1

• $hash('foo')is a variable function.
  $hashmay contain a string with the name of the function or an anonymous function.

  $hash = 'md5';
  
  // This means echo md5('foo');
  // Output: acbd18db4cc2f85cedef654fccc4a4d8
  echo $hash('foo');
  

• $$foois a variable variable.
  $foomay contain a string with the name of the variable.

  $foo = 'bar';
  $bar = 'baz';
  
  // This means echo $bar;
  // Output: baz
  echo $$foo;
  

• $bar[$foo]is a variable array.
  $foocan contain everything that can be used as an array key, for example, a numerical index or an associative name.

  $bar = array('first' => 'A', 'second' => 'B', 'third' => 'C');
  $foo = 'first';
  
  // This tells PHP to look for the value of key 'first'
  // Output: A
  echo $bar[$foo];
  

PHP , ( ).