PHP Registry Pattern

Question

I've found the piece of code below in several places around the web and even here on Stack Overflow, but I just can't wrap my head around it. I know what it does, but I don't know how it does it even with the examples. Basically it's storing values, but I don't know how I add values to the registry. Can someone please try to explain how this code works, both how I set and retrieve values from it?

class Registry {

    private $vars = array();

    public function __set($key, $val) {
        $this->vars[$key] = $val;
    }

    public function __get($key) {
        return $this->vars[$key];
    }
}

Solution 1

It's using PHP's hacked on property overloading to add entries to and retrieve entries from the private $vars array.

To add a property, you would use...

$registry = new Registry;
$registry->foo = "foo";

Internally, this would add a foo key to the $vars array with string value "foo" via the magic __set method.

To retrieve a value...

$foo = $registry->foo;

Internally, this would retrieve the foo entry from the $vars array via the magic __get method.

The __get method should really be checking for non-existent entries and handle such things. The code as-is will trigger an E_NOTICE error for an undefined index.

A better version might be

public function __get($key)
{
    if (array_key_exists($key, $this->vars)) {
        return $this->vars[$key];
    }

    // key does not exist, either return a default
    return null;

    // or throw an exception
    throw new OutOfBoundsException($key);
}

Solution 2

You might want to check out PHP.NET - Overloading

Basically, you would do...

$Registry = new Registry();

$Registry->a = 'a'; //Woo I'm using __set
echo $Registry->a; //Wooo! I'm using __get

So here, I'm using __set($a, 'This value is not visible to the scope or nonexistent') Also, I'm using __get($a);

Hope this helped!

Comments

[Ian Hoar]

Thanks, I thought I was going crazy. This helped me go back and check what was going on. I was trying to set a form action, but it was always coming up blank. Turns out I was on the index page, so there was no value. DOH!. I tried the form on another page and it worked fine. Thanks for your help. When you say hacked on, does that mean it shouldn't be used this way?

[Phil]

@IanHoar Well, it definitely shouldn't be called overloading

[Wolfpack'08]

@Phil I can see how you add the key, but how do you add a key-value pair?

[Phil]

@Wolfpack Not sure what you mean. When you set $registry->key = 'value', internally it stores a key / value pair of key and value respectively but to the outside world, they appear as class properties

[Wolfpack'08]

$foo = $registry->foo; ; $foo = $registry->foo = 'bar';? I don't full understand the purpose of this code, honestly. I'm just reading articles.

[Phil]

@Wolfpack still not sure what your question is. Your first snippet uses the __get() method to retrieve property foo and assign the value to local var $foo. Your second assigns 'bar' to both the property foo and local variable $foo. It's no different to $a = $b = 'some value'

[Wolfpack'08]

@Phil I was just following the template: $registry->key = 'value'