Are there any good use-cases for variable variables?

Question

Today I encountered this little PHP gem called variable variables.

$literal = "Hello";
$vv = "literal";
echo $$vv; // => prints "Hello"

Are there actually any real use-cases for this language feature?

Answer 1

Assuming a url of http://example.com/hello/world, and the following .htaccess:

RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ router.php?/$1 [L]

router.php, an overly simplistic router, would be:

<?php

$request = explode("/", $_SERVER["REQUEST_URI"]);

$controllerClass = $request[0];
$controllerActionMethod = $request[1];

$controller = new $controllerClass();
$controller->$controllerActionMethod(); // hello::world();

?>

That's a pretty standard practical use of variable variables. Neat trick, but you should be extremely careful when using it, over/abusing it will certainly lead to horribly unmaintainable code.

Disclaimer: The code presented in the answer is only intended to illustrate the use of the feature. It does not cover proper design, security, maintainability, or sanity. Do not use.

Answer 2

Honestly? In nearly 10 years of PHP experience I've never encountered a problem to which the only solution (or indeed the best solution from a choice) was to use a variable variable. The closest I've got is being able to access constants on an instance of a class such as $classInstance::CLASS_CONSTANT (which has been available since PHP 5.3) and even then I don't normally have much use for it.

I suspect it's one of the many warts that exist in PHP that seemed like a good idea at the time.

So in answer to your question, if there are use cases where it's the best or only approach to a particular problem, I've not seen them.