variable-variables in PHP

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

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

• $$foo is a variable variable.
  $foo may contain a string with the variable name.

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

• $bar[$foo] is a variable array key.
  $foo may contain anything that can be used as an array key, like a numeric 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];
  

The PHP manual has an article on variable variables, and an article on anonymous functions (but I didn't show an example above for the latter).