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 is left is then $"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 inside quoted strings in PHP:

echo "Hello ${$var}";

This syntax is also used to help 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.

Dynamic variables may not initially 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 this:

$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 can be handy to turn these entity names into real PHP variables, so you can refer to them as simply $abc and $def. This can be done as follows:

while(list($index,$value) = each($array)) {
  $$index = $value;
}