PHP ambiguous statement when setting dynamic property value of class

I have a class called PreferenceCollection, which I load for a user into $_SESSION['Preferences'] once they log in to my web application. This simple class uses magic methods for getting and setting values. Here are the relevant parts:

class PreferenceCollection {
    private $prefs;

    function __construct() {
        $this->prefs=Array();
    }

    function __get($var) {
        return $this->prefs[$var];
    }

    function __set($var, $value) {
        $this->prefs[$var]=$value;
        $this->save();
    }
}

Later on in my code, I have found it necessary to set the value of a dynamically chosen property. For this example, $key='pref_some_preference' and value='something'.

$_SESSION['Preferences']->substr($key, 5)=$value;

What I expect is the equivalent of $_SESS开发者_StackOverflow中文版ION['Preferences']->some_preference=$value. That is, the __set magic method will be called with the first parameter of some_preference, and the second parameter, $value.

Instead, this line gives me a fatal error:

PHP Fatal error: Can't use method return value in write context

I assume that PHP is interpreting that I want to set the return value of the substr() call to something, rather than setting the property.

Is my interpretation of what is happening correct? How would I get around this problem?

For now I am working around the issue with a public set method, but am curious how I could set that property directly.

I think you want

$_SESSION['Preferences']->{substr($key, 5)} = $value;

The braces {} are required to guide the PHP parser into the correct direction ;-) Given that $key is 12345test this would call __set('test', $value).