add_action( 'init', function() {
    // do something here
} );

Using a function block to hook a set of instructions. With the init hook, the set of instructions will be executed right after wordpress has finished loading the necessary components.

function my_init_function() {
    // do something here
}

add_action( 'init', 'my_init_function' );

Using the name of the function to hook a set of instructions. With the init hook, the set of instructions will be executed right after wordpress has finished loading the necessary components.

class MyClass {
    static function my_init_method() {
        // do something here
    }
}

add_action( 'init', array( 'MyClass', 'my_init_method' ) );

Using a static method of a class to hook a set of instructions. With the init hook, the set of instructions will be executed right after wordpress has finished loading the necessary components.

class MyClass {
    function my_init_method() {
        // do something here
    }
}

$obj = new MyClass();

add_action( 'init', array( $obj, 'my_init_method' ) );

Using a method of an object to hook a set of instructions. With the init hook, the set of instructions will be executed right after wordpress has finished loading the necessary components.