The final keyword is used to restrict the inheritance of a class or the overriding of a method. 

Here's a simple explanation:

1.  Final Classes:

When a class is declared as final, it cannot be extended or subclass.

Example:

<?php
final class ParentClass {
    // Class definition
}

// This will cause a fatal error
class ChildClass extends ParentClass {
    // Class definition
}

?>

2. Final Methods:

• When a method is declared as final within a class, it cannot be overridden by child classes.
• This means that you cannot redefine or override a final method in a subclass.

<?php
class ParentClass {
    final public function someMethod() {
        // Method definition
    }
}

class ChildClass extends ParentClass {
    // This will cause a fatal error
    public function someMethod() {
        // Method definition
    }
}

?>

NOTE: The final keyword in PHP is used to prevent inheritance or overriding.