How to use variable variables in PHP

One of the biggest time-savers in PHP is the ability to use variable variables.  While often intimidating for newcomers to PHP, variable variables are extremely powerful once you get the hang of them.

Variable variables are just variables whose names can be programatically set and accessed.  For example, the code below creates a variable called $hello and outputs the string “world”.  The double dollar sign declares that the value of $a should be used as the name of newly defined variable.

<?php
$a = 'hello';
$$a = 'world'
echo $hello;
?>

When I started with PHP about 10 years ago, everyone was still using global variables.  That meant that anything you passed as a GET variable could be used as a local variable.  It was very convenient, but unfortunately not very secure.  For me, typing $HTTP_GET_VARS[‘count’] just wasn’t as fun as being able to use $count.  I found myself adding long declaration lists to the top of my files that did nothing but convert my GET/POST variables to local variables.  My code started to look like this:

<?php
$salutation = $HTTP_GET_VARS['salutation'];
$fname = $HTTP_GET_VARS['fname'];
$lname = $HTTP_GET_VARS['lname'];
$email = $HTTP_GET_VARS['email'];
...
?>

Do that for a couple dozen variables and you’ll start telling yourself there has to be a better way.  Nowadays you can use $_GET instead of $HTTP_GET_VARS, but the better solution is to use variable variables. Now my code looks more like this:

<?php
// create an array of all the GET/POST variables you want to use
$fields = array('salutation','fname','lname','email','company','job_title','addr1','addr2','city','state',
                'zip','country','phone','work_phone');

// convert each REQUEST variable (GET, POST or COOKIE) to a local variable
foreach($fields as $field)
    ${$field} = sanitize($_REQUEST[$field]);
?>

This has several benefits.  I reduced 14 lines of code down to 3.  I now have one place to sanitize all my external input. And if I ever decide to change a variable name, I have one less place in my code to fix.

This benefit of this technique increases as you use the $fields array throughout your code.  I now utilize the $fields array when saving my form data to the database.  I use it for loading existing user values from the database.  I use it for passing my form fields back to smarty:

<?php
$form = array();
foreach($fields as $field)
    $form[] = $_REQUEST[$field];
$smarty->assign('form',$form);
?>

Variable variables have become one of my favorite features of PHP. They’ve allowed me to tighten up a lot of my code and made it a lot more maintainable.

Have you done anything cool with variable variables?  What other PHP tricks have revolutionized the way you write code?