What does parent::functionname() mean in PHP and when should it be used?

The `parent::functionname()` syntax in PHP is used to call a method from the parent class within a child class. This is useful when you want to extend the functionality of a method in the parent class without completely overriding it. By using `parent::functionname()`, you can access the parent class method and then add additional functionality to it in the child class.

class ParentClass {
    public function someMethod() {
        echo "Parent method";
    }
}

class ChildClass extends ParentClass {
    public function someMethod() {
        parent::someMethod(); // Calls the method from the parent class
        echo "Child method";
    }
}

$child = new ChildClass();
$child->someMethod();
```

Output:
```
Parent method
Child method