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

 

In PHP 5, self and $this are used to access class members (properties and methods), but they have different purposes and usage contexts:

1. self:

   • self is a keyword used to refer to static members (properties and methods) within a class.
   • It's used to access class-level members that are not tied to a specific instance of the class.
   • You typically use self within the class definition itself, to access static properties and call static methods.

2. $this:

   • $this is a variable used to refer to instance members (properties and methods) within a class.
   • It's used to access instance-level members, which are specific to each object created from the class.
   • You use $this within class methods to refer to instance properties and call instance methods.

Here's an example that illustrates the difference:

php

class MyClass {
    public static $staticProperty = "Static Property";
    public $instanceProperty = "Instance Property";

    public static function staticMethod() {
        // Using 'self' to access a static property
        return self::$staticProperty;
    }

    public function instanceMethod() {
        // Using '$this' to access an instance property
        return $this->instanceProperty;
    }
}

// Accessing static property and method using 'self'
echo MyClass::$staticProperty . "\n"; // Output: "Static Property"
echo MyClass::staticMethod() . "\n";   // Output: "Static Property"

// Creating an instance of MyClass
$obj = new MyClass();

// Accessing instance property and method using '$this'
echo $obj->instanceProperty . "\n"; // Output: "Instance Property"
echo $obj->instanceMethod() . "\n";  // Output: "Instance Property"

In this example:

• self::$staticProperty and self::staticMethod() are used to access static members within the class, which don't depend on object instances.

• $this->instanceProperty and $this->instanceMethod() are used to access instance members, which are specific to individual objects created from the class.

In PHP 5, you should use self for static members and $this for instance members based on the context in which you need to access class members. Starting from PHP 7, you can also use ::class to reference the class name, which can be useful for resolving class names in a more dynamic way.