php class and variables

Question

Say I have a class

class person {
    public static function get_pk(){
        return self::$primary_key;
    }
}

and an extended class

class user extends person {
    protected $primary_key = 'id';
}

and I want to run:

echo user::get_pk(); // should echo id

Whenever I run this it fails naturally since $primary_key is not a static variable. How do I get it to return the primary key?

Yes, I can change the class structure, function structure and make it non-static, but for all intents and purposes assume the only thing we can change is the content of the static function get_pk

You will need to declare a variable of the class to access the variable. It is not available without reference. — Kami
– Kami, Commented Dec 6, 2012 at 17:54
The real question is, why are $primary_key and get_pk() static in person? That doesn't make a whole lot of sense if you ask me. — NullUserException
– NullUserException, Commented Dec 6, 2012 at 17:59
@NullUserException It makes sense in the global context of my projects, but obviously when showing just the 'breaking' part it won't make much sense =) — Aram Papazian
– Aram Papazian, Commented Dec 6, 2012 at 18:51
@Kami What do you mean exactly? Do you mean to instantiate a class within the class and then pull the variable? $class = get_called_class(); $obj = new $class; return $obj->$primary_key; ... Like that? (Although I'd prefer not to create a whole instantiation just for 1 variable =/) — Aram Papazian
– Aram Papazian, Commented Dec 6, 2012 at 18:52

Geo · Accepted Answer · 2012-12-06 19:09:40Z

1

The only way I can see this done is by declaring the $primary_key variable in the parent class and then assigning a value to it outside of the child class definition scope. (Notice, I've added abstract operators to indicate that the classes are static and capitalized the first letters of the class names to follow the convention.)

<?

abstract class Person {
    public static $primary_key;
    public static function get_pk() {
        return self::$primary_key;
    }
}

abstract class User extends Person {
}

User::$primary_key = 'id';
echo User::get_pk();

/* End of file */

UPDATE

Aram, if you want the $primary_key to stay non-static then the method will need to be non-static as well as "the pseudo-variable $this is not available inside the method declared as static" as per the Static Keyword manual.

Alternatively you could convert your code to use instantiated class objects:

<?

class Person {
    public function get_pk() {
        return $this->primary_key;
    }
}

class User extends Person {
    protected $primary_key = 'id';
}

$user = new User();
echo $user->get_pk();

/* End of file */

edited Dec 6, 2012 at 19:09

answered Dec 6, 2012 at 18:15

Geo

13.1k4 gold badges46 silver badges56 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Aram Papazian Over a year ago

This won't work because I want $primary_key to stay non-static

Aram Papazian Over a year ago

My point is I want $primary_key to stay non-static and the function get_pk to stay static... That is the issue I am facing. I know I can easily change the static nature of either of those to make it work, but that won't help me with the code or answer the question...

Geo Over a year ago

according to the PHP manual (link above) this is impossible. But I'll be curious to see if you find a work around ;-) Good luck!

Aram Papazian Over a year ago

Yay for being impossible! If I find something that works, I'll let you guys know =) Thanks for all the help.

deceze · Accepted Answer · 2012-12-06 17:52:21Z

1

You cannot access non-static properties from static methods, since for the properties to be created the class needs to be instantiated. You can introspect the class to get values of its definition though:

public static function get_pk() {
    $class = new ReflectionClass(get_called_class());
    $props = $class->getDefaultProperties();
    return $props['primary_key'];
}

But really, this is pretty messy and I would not recommend this for production use. Structure your classes properly to be either static or not static.

answered Dec 6, 2012 at 17:52

deceze♦

525k89 gold badges807 silver badges954 bronze badges

Comments

Ed Heal · Accepted Answer · 2012-12-06 17:52:07Z

0

Static variables/methods relate to the class i.e. does not require an object to be created. Your primary_key variable is created for each object. So the static method is unable to access it because the static method has no notion of a particular object and therefore does not know anything about the primary_key. Static methods do not have the variable $this (hence self) defined.

I guess your best bet is to review static keyword.

answered Dec 6, 2012 at 17:52

Ed Heal

60.3k18 gold badges91 silver badges137 bronze badges

Collectives™ on Stack Overflow

php class and variables

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related