When to use self over $this?

Question

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

When is each appropriate?

Answer 1

DO NOT USE self::, use static::

There is another aspect of self:: that is worth mentioning. Annoyingly self:: refers to the scope at the point of definition not at the point of execution. Consider this simple class with two methods:

class Person
{

    public static function status()
    {
        self::getStatus();
    }

    protected static function getStatus()
    {
        echo "Person is alive";
    }

}

If we call Person::status() we will see "Person is alive" . Now consider what happens when we make a class that inherits from this:

class Deceased extends Person
{

    protected static function getStatus()
    {
        echo "Person is deceased";
    }

}

Calling Deceased::status() we would expect to see "Person is deceased" however what we see is "Person is alive" as the scope contains the original method definition when call to self::getStatus() was defined.

PHP 5.3 has a solution. the static:: resolution operator implements "late static binding" which is a fancy way of saying that it's bound to the scope of the class called. Change the line in status() to static::getStatus() and the results are what you would expect. In older versions of PHP you will have to find a kludge to do this.

See PHP Documentation

So to answer the question not as asked ...

$this-> refers to the current object (an instance of a class), whereas static:: refers to a class