Variable variables seem pretty cool, but I can't think of a scenario where one would actually use them in a production environment. Have you done so? How did you use them?
Its purpose, I guess, is to allow novice programmers to dynamically change data without using "complicated stuff" like composite types (arrays and objects).
Seems like something something Rasmus would consider a good idea.
I never use them.
Think of it for use in a template system where you are using php files and need to set in variables:
function fetch_template($file, $vars){
$ret = 'File not loaded.';
if(file_exists(TEMPLATE_PATH.$file)){
//could do this with extract() but I am showing you
foreach($vars as $varName => $value){
${$varName} = $value;
}
ob_start();
include(TEMPLATE_PATH.$file);
$ret = ob_get_contents();
ob_end_clean();
}
return $ret;
}
Now assuming you used these variable names in your template, you could call it pass variables into it for use.
echo fetch_template('hi_there.tpl', array('name'=>'JJ'));
Then in your template:
Hello <?php echo $name; ?>!
Personally, I use them fairly often. All calls of the following types use variable-variables:
$foo->$bar = 'test';
$foo->$bar();
$bar();
So any time you do a dynamic method/function call, you're using variable-variables...
A common use for this is accessing protected properties via the __get
magic method. I've seen the following quite often:
public function __get($name) {
return isset($this->$name) ? $this->$name : null;
}
Which by definition is using variable variables to provide read-access to the protected members...
I've never directly used the $$var
syntax (and don't think I ever will). I have seen it used to access global variables by name global $$name; echo $$name;
, but the same thing can be done with the $_GLOBALS[$name]
syntax, so that's not a good use-case (not to mention that using global variables is usually seen as bad practice)...
I found useful in single scenario . I was having youtube API results in json format like this
$obj->media$title => Video title
So i used like
$mt = 'media$title';
$obj->$mt ;
So worked for me here :)
A variable variable is essentially an array (map/dictionary). The following are equivalent ideas:
<?php
$foo = array('a' => 1);
$bar = 'a';
echo $foo[$bar]."\n";
$foo_a = 1;
$bar = 'a';
$vv = "foo_$bar";
echo $$vv."\n";
?>
Thus if you wrap your "variable variables" into a parent array, you can do away with them.
I've seen people use variable properties inside classes:
<?php
class Foo
{
private $a = 1;
public function __get($key)
{
if (isset($this->$key)) return $this->$key;
}
}
$foo = new Foo();
echo $foo->a;
?>
But again, you could use an array:
<?php
class Foo
{
private $props = array('a' => 1);
public function __get($key)
{
if (array_key_exists($key, $this->props))
return $this->props[$key];
}
}
$foo = new Foo();
echo $foo->a;
?>
And outside classes:
<?php
class Foo
{
public $a = 1;
}
$foo = new Foo();
$prop = 'a';
echo $foo->{$prop};
?>
So you never "have" to use variable variables or variable properties when writing your own controlled code. My personal preference is to never use variable variables. I occasionally use variable properties, but prefer to use arrays when I'll be accessing data in that way.