php-Dynamic Variables

Sometimes it is useful to set and use variables dynamically. 
Normally, you assign a variable like this: 

$var = "hello";
Now let's say you want a variable whose name is the 
value of the $var variable. You can do that like this:

$$var = "World";
PHP parses $$var by first dereferencing the innermost 
variable, meaning that $var becomes "hello". The 
expression that's left is $"hello", which is just $hello.
 In other words, we have just created a new variable 
named hello and assigned it the value "World"
You can nest dynamic variables to an infinite level in 
 PHP, although once you get beyond two levels, it 
can be very confusing for someone who is trying to read your code.
There is a special syntax for using dynamic variables, and 
any other complex variable, inside quoted strings in PHP: 

echo "Hello ${$var}";
This syntax also helps resolve an ambiguity that occurs when
 variable arrays are used. Something like $$var[1] is 
ambiguous because it is impossible for PHP to know which
 level to apply the array index to. ${$var[1]} tells 
PHP to dereference the inner level first and apply the 
 array index to the result before dereferencing the outer level.
 ${$var}[1], on the other hand, tells PHP to apply the index
 to the outer level.
Initially, dynamic variables may not seem that useful, but there
 are times when they can shorten the amount of code you need 
to write to perform certain tasks. For example, say you have 
an associative array that looks like: 

$array["abc"] = "Hello";
$array["def"] = "World";
Associative arrays like this are returned by various functions
 in the PHP modules. mysql_fetch_array() is one example.
 The indices in the array usually refer to fields or entity names 
within the context of the module you are working with. It's
 handy to turn these entity names into real PHP variables, 
so you can refer to them as simply $abc and $def