In object-oriented PHP programming, why is it important to encapsulate validation and restrictions within the class itself rather than externally?

Encapsulating validation and restrictions within the class itself ensures that the class is responsible for maintaining its own integrity. This approach promotes better organization, reusability, and maintainability of the code. By encapsulating these rules within the class, you can easily enforce them whenever an object of that class is created or modified. 

class User {
    private $username;

    public function setUsername($username) {
        if(strlen($username) < 5) {
            throw new Exception("Username must be at least 5 characters long");
        }
        $this->username = $username;
    }
}

$user = new User();
$user->setUsername("john"); // This will throw an exception

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