PHP has a legacy concept called "variable variables". This is an older, more limited programming concept that came before composite data structures were available. Since the PHP language now supports composite data structures, the concept of variable variables is essentially obsolete.
The PHP manual states: 
    "Sometimes it is convenient to be able to have variable variable names. That is, a variable name which can be set and used dynamically."
This approach has historically been used in programming languages that do not support composite data structures. There is no programmatic function or algorithm in PHP that can be obtained with variable that cannot also be obtained with composite data structures. 
Moreover, "variable variables" are error-prone and require more maintenance overhead.
Data structures are the way to represent composite entities using regular PHP variables.
Those familiar with database design and database implementation know about the concept of database normalization.
Data structures in PHP represent a similar concept. Whenever dealing with complex concepts and representing them in PHP, data structures are a way to normalize PHP variables to consistently and uniformly represent complex concepts.
String Example:
 $person_name = 'Alice';
Array Examples:
 $person_names = Array( 0=> 'Alice', 1=> 'Bob', 2=> 'Charlie',  );
 $alice_info   = Array( 0=> 'Alice', 1=> 'Female', 2=> '26', 3=> 'alice@example.com',  );
The square brackets method allows you to set up by directly setting the values. For example, to make $foobar[1] = $foo, all you need to do is:
SimpleDictionary Examples:
 $person_names = Array( 'person1'=> 'Alice', 'person2'=> 'Bob', 'person3'=> 'Charlie',  );
 $alice_info   = Array( 'first_name'=> 'Alice', 'sex'=> 'Female', 'age'=> '26', 'email'=> 'alice@example.com',  );
SimpleDictionary Examples:
$user_profile = Array(
  main => Array(
    first_name  => "Archibald",
    last_name   => "Shaw",
    sex         => "male",
    age         => "33",    
  ),
  guardian => Array(
    first_name => "",
    last_name => "",
  ),
  children => Array(
    0 => Array(
      first_name => "Sally",
      last_name => "Shaw",
    ),
    1 => Array(
      first_name => "Scott",
      last_name => "Shaw",
    ),
  ),
  
);
http://stackoverflow.com/questions/3523670/whats-an-actual-use-of-variable-variables