Laravel/eloquent isDirty() & getDirty()

Intro

I’ve done a quick test to exercise a new-to-me part of the Laravel (5.6) Eloquent API:

$model->isDirty();
$model->isDirty("field_name");
$model->isDirty(["field_name", "another_field_name"]);

$model->getDirty();

I want to know when ->isDirty() returns TRUE…

My model

I’ve got an Eloquent model for my contacts table:

namespace App\Contacts;
use Illuminate\Database\Eloquent\Model;

class Contact extends Model {
    // ...
}

Testing…

$contact = new Contact;
echo($contact->isDirty());
//  false

$contact->first_name = "Mek";

echo($contact->isDirty());
//  true

echo($contact->isDirty("last_name"));
//  false

echo($contact->isDirty(["first_name", "last_name"]));
//  true

$contact->save();
echo($contact->isDirty());
//  false

• isDirty() seems always to return boolean TRUE/FALSE regardless of what attributes you pass: whether you want to check globally (no attributes == any field), for changes to one field (string attribute == field name), or for changes to any number of fields (array attribute == array of field names)
• Works predictably, as expected, whether I use a model instance corresponding to a new record or an existing record.
• Also (extra test, not shown here): if I change a field to a different value, $contact->isDirty() returns TRUE; but if I “change” a field to the same value it had before, $contact->isDirty() returns FALSE – which is good: it’s not returning false positives.