When to use self over $this?

Question 1

In PHP 5, what is the difference between using self and $this?

When is each appropriate?

Question 2

Short Answer

Use $this to refer to the current object. Use self to refer to the current class. In other words, use $this->member for non-static members, use self::$member for static members.

Full Answer

Here is an example of correct usage of $this and self for non-static and static member variables:

<?php
class X {
    private $non_static_member = 1;
    private static $static_member = 2;

    function __construct() {
        echo $this->non_static_member . ' '
           . self::$static_member;
    }
}

new X();
?>

Here is an example of incorrect usage of $this and self for non-static and static member variables:

<?php
class X {
    private $non_static_member = 1;
    private static $static_member = 2;

    function __construct() {
        echo self::$non_static_member . ' '
           . $this->static_member;
    }
}

new X();
?>

Here is an example of polymorphism with $this for member functions:

<?php
class X {
    function foo() {
        echo 'X::foo()';
    }

    function bar() {
        $this->foo();
    }
}

class Y extends X {
    function foo() {
        echo 'Y::foo()';
    }
}

$x = new Y();
$x->bar();
?>

Here is an example of suppressing polymorphic behaviour by using self for member functions:

<?php
class X {
    function foo() {
        echo 'X::foo()';
    }

    function bar() {
        self::foo();
    }
}

class Y extends X {
    function foo() {
        echo 'Y::foo()';
    }
}

$x = new Y();
$x->bar();
?>

The idea is that $this->foo() calls the foo() member function of whatever is the exact type of the current object. If the object is of type X, it thus calls X::foo(). If the object is of type Y, it calls Y::foo(). But with self::foo(), X::foo() is always called.

From http://www.phpbuilder.com/board/showthread.php?t=10354489:

By http://board.phpbuilder.com/member.php?145249-laserlight

Question 3

You can't define a class in another class. You should include files with other classes outside of the class. In your case, that will give you two top-level classes db and some. Now in the constructor of some you can decide to create an instance of db. For example:

include SITE_ROOT . 'applicatie/' . 'db.class.php';

class some {

    public function __construct() {
        if (...) {
            $this->db = new db;
        }
    }

}

Question 4

include global $myarray at the start of setvalue() function.

public function setvalue() {
    global $myarray;
    $myvalue = $myarray[0];
}

UPDATE:
As noted in the comments, this is bad practice and should be avoided.
A better solution would be this: https://.com/a/17094513/3407923.

Question 5

It is idiomatic to prefer to omit self. when invoking methods; it is generally never needed.
You must use self.foo = xxx when calling a setter method, instead of foo = xxx, so that Ruby realizes that you are not trying create a new local variable.
- Similarly, in the unlikely event that you have an existing local variable do_something with the same name as a method, you must use self.do_something to invoke the method, as just do_something will end up reading the variable.
You cannot use self.foo(...) to call a private method; you must instead call just foo(...).

Question 6

When you're doing an action on the instance that's calling the method, you use self.

With this code

class SocialData < ActiveRecord::Base
  def set_active_flag(val)
    active_flag = val
    save!
  end
end

You are defining a brand new scoped local variable called active_flag, setting it to the passed in value, it's not associated with anything, so it's promptly thrown away when the method ends like it never existed.

self.active_flag = val

However tells the instance to modify its own attribute called active_flag, instead of a brand new variable. That's why it works.

When to use self over $this?

Answers

Short Answer

Full Answer

php,class,oop,scope