Variables can be accessed via dynamic variable names. The name of a variable can be stored in another variable, allowing it to be accessed dynamically. Such variables are known as variable variables.

To turn a variable into a variable variable, you put an extra $ put in front of your variable.

$variableName = 'foo';
$foo = 'bar';

// The following are all equivalent, and all output "bar":
echo $foo;
echo ${$variableName};
echo $$variableName;

//similarly,
$variableName  = 'foo';
$$variableName = 'bar';

// The following statements will also output 'bar'
echo $foo; 
echo $$variableName; 
echo ${$variableName};

Variable variables are useful for mapping function/method calls:

function add($a, $b) {
    return $a + $b;
}

$funcName = 'add';

echo $funcName(1, 2); // outputs 3

This becomes particularly helpful in PHP classes:

class myClass {
    public function __construct() {
        $functionName = 'doSomething';
        $this->$functionName('Hello World');
    }

    private function doSomething($string) {
        echo $string; // Outputs "Hello World"
    }
}

It is possible, but not required to put $variableName between {}:

${$variableName} = $value;

The following examples are both equivalent and output “baz”:

$fooBar = 'baz';
$varPrefix = 'foo';

echo $fooBar;               // Outputs "baz"
echo ${$varPrefix . 'Bar'}; // Also outputs "baz"

Using {} is only mandatory when the name of the variable is itself an expression, like this:

${$variableNamePart1 . $variableNamePart2} = $value;

It is nevertheless recommended to always use {}, because it’s more readable.

While it is not recommended to do so, it is possible to chain this behavior:

$$$$$$$$DoNotTryThisAtHomeKids = $value;

It’s important to note that the excessive usage of variable variables is considered a bad practice by many developers. Since they’re not well-suited for static analysis by modern IDEs, large codebases with many variable variables (or dynamic method invocations) can quickly become difficult to maintain.

Differences between PHP5 and PHP7

Another reason to always use {} or (), is that PHP5 and PHP7 have a slightly different way of dealing with dynamic variables, which results in a different outcome in some cases.

In PHP7, dynamic variables, properties, and methods will now be evaluated strictly in left-to-right order, as opposed to the mix of special cases in PHP5. The examples below show how the order of evaluation has changed.

Case 1 : $$foo['bar']['baz']

• PHP5 interpretation : ${$foo['bar']['baz']}
• PHP7 interpretation : ($$foo)['bar']['baz']

Case 2 : $foo->$bar['baz']

• PHP5 interpretation : $foo->{$bar['baz']}
• PHP7 interpretation : ($foo->$bar)['baz']

Case 3 : $foo->$bar['baz']()

• PHP5 interpretation : $foo->{$bar['baz']}()
• PHP7 interpretation : ($foo->$bar)['baz']()

Case 4 : Foo::$bar['baz']()

• PHP5 interpretation : Foo::{$bar['baz']}()
• PHP7 interpretation : (Foo::$bar)['baz']()