PHP Child class accessing Parent variable problem?

If you mean to make it impossible to create a Username, make the Username class abstract. Also, do NOT make a new contact information from the parent class. Here's another way to put it: abstract class Username { protected $id; protected $username; public __construct($id) { $this->id = (int) $id; // Forces it to be a string } } class ContactInformation extends Username { protected $mobile; protected $email; protected $nextel_id; public __construct($id, $mobile, $email, $nextel_id) { parent::__construct($id) $this->mobile = $mobile; .... } } Now, instead of instantiating the Username directly (which is now impossible), you instead create a ContactInformation.

ContactInformation then calls the Username constructor in its own constructor.

Declaring a protected constructor is common in singletons. But you're probably right about using abstract in this case, so you'd get more useful errors if you tried to create a new Username(); – James Socol May 6 '09 at 20:55 Yes, it is true that it is common in the Singleton-pattern. But the Singleton-pattern is not great to use when you need to extend the Singleton-class, especially not in PHP.

A lot of people also argue that the Singleton-pattern is flawed, and that it is better to ensure that, instead of forcing a single instance programmatically, only a single instance is created in the application. Also, if he wants to follow the Singleton-pattern he will need a "getInstance"-method and also to make the __clone-magic method private. – PatrikAkerstrand May 6 '09 at 21:46 I don't think James was suggesting that Singleton is applicable here; he was pointing out that a private or protected constructor has valid uses.

– Rob May 6 '09 at 22:38.

The parent:: method is only used to access parent methods that you have overridden in your subclass, or static variables like: class Base { protected static $me; public function __construct () { self::$me = 'the base'; } public function who() { echo self::$me; } } class Child extends Base { protected static $me; public function __construct () { parent::__construct(); self::$me = 'the child extends '. Parent::$me; } // until PHP 5.3, will need to redeclare this public function who() { echo self::$me; } } $objA = new Base; $objA->who(); // "the base" $objB = new Child; $objB->who(); // "the child extends the base" You probably want a proper subclass. Don't create a subclass in the constructor of the base class, that turns all sorts of OOP best-practices upside down (loose coupling, etc) while also creating an infinite loop.(new ContactInformation() calls the Username constructor which creates a new ContactInformation() which...).

If you want a subclass, something like this: /** * Stores basic user information */ class User { protected $id; protected $username; // You could make this protected if you only wanted // the subclasses to be instantiated public function __construct ( $id ) { $this->id = (int)$id; // cast to INT, not string // probably find the username, right? } } /** * Access to a user's contact information */ class ContactInformation extends User { protected $mobile; protected $email; protected $nextel; // We're overriding the constructor... public function __construct ( $id ) { // ... so we need to call the parent's // constructor. Parent::__construct($id); // fetch the additional contact information } } Or you could use a delegate, but then the ContactInformation methods wouldn't have direct access to the Username properties.

Class Username { protected $id; protected $contact_information; public function __construct($id) { $this->id = (int)$id; $this->contact_information = new ContactInformation($this->id); } } class ContactInformation // no inheritance here! { protected $user_id; protected $mobile; public function __construct($id) { $this->user_id = (int)$id; // and so on } }.

First, when you create a ContactInformation, it is also contains all the non-private properties and methods of Username. You do not need a separate Username instance. Class Username { protected $id; protected $username; protected __construct($id) { $this->id = (int) $id; // Forces it to be a string } } Class ContactInformation extends Username { protected $mobile; protected $email; protected $nextel_id; // Pretend that these are here because they're defined in my parent //protected $id; //protected $username; public __construct($id) { parent::__construct($id); echo $this->id; //Should echo 1 } } However, since all the fields are protected, this isn't going to work: $contact_information = new ContactInformation(1); // Works fine echo $contact_information->id; // Whoops, visibility error because id isn't public.

What I want is: access the members from the Parent Class on the Child class (thats include the variables). I do need to query some stuff and I need to access all the properties from the Parent class (and their current values) in order to make my class work as expected. Also I have some other classes that derives from the Parent class.

Jonathan got my point and it's working correctly. I want to do exactly what jonathan just explained (and it works), but maybe theres a better way to do this?

If this is correct, here's how its done: class A { // These are the properties you want to access from the child object public $property_a; public $property_b; public $property_c; // This is the child object variable public $child_object; public function __construct( ) { // Pass 'this' into the child so that the child has a reference back to the parent $this->child_object = new B($this); } } class B { // Holds a reference to the parent object protected $parent_object; public function __construct( $object ) { // Remember the reference to the parent object $this->parent_object = $object; } // Just a Demonstration Method public print_parent_property_a() { // Reach into the referred parent object, and get it's property print $this->parent_object->property_a; } } So if you were to do: $my_object = new A(); $my_object->property_a = 'test_value'; $my_object->child_object->print_parent_property_a(); You'd get 'test_value' It's slightly different from your example, in that you'll need the parent class properties to be public so that the child can access them. This all works because in PHP, objects are always passed by reference unless you explicity clone them.

You've used both composition and inheritance in this case; the latter is unnecessary. Can you clarify your example? – Rob May 6 '09 at 22:39.

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Caveat: Might not be the most efficient way to do it: SELECT FROM mytable WHERE ORDER BY date DESC LIMIT 1 UNION SELECT FROM mytable WHERE ORDER BY date ASC LIMIT 1.

1 I think the 'Top' keyword is for SQL server only, MySQL/Postgre uses 'Limit' – Robo Sep 28 '09 at 4:28 You are correct. I will edit mine and vote up yours. – Mitch Wheat Sep 28 '09 at 4:30 Good on yer, Mitch.

I was wondering how many upvotes I could get if I posted an incorrect answer. – pavium Sep 28 '09 at 4:36 Using UNION ALL will make this marginally faster, as it removes a check for duplicates. It'll differ in how it works if the first and last row are the same of course - UNION will return just one row, UNION ALL will return the same row twice.

– Magnus Hagander Sep 28 '09 at 8:16 @Magnus Hagander: I'm not sure it will be any faster when there is at most 2 rows. Granted, I would normally make the distinction between UNION and UNION ALL. – Mitch Wheat Sep 28 '09 at 8:54.

First record: SELECT FROM mytable WHERE ORDER BY date ASC LIMIT 1 Last record: SELECT FROM mytable WHERE ORDER BY date DESC LIMIT 1.

1 Not every task must be done in a single query. – Bill Karwin Sep 28 '09 at 4:47 The UNION ALL method mentioned in the other comment will definitely be faster than issuing two queries. – Magnus Hagander Sep 28 '09 at 8:16.

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