Immutables.php

<?php

namespace App\Models\Traits;

use LogicException;

trait Immutables
{
    /**
     * Set a given attribute on the model.
     *
     * @override \Illuminate\Database\Eloquent\Model
     *
     * @param  string  $key
     * @param  mixed  $value
     * @return $this
     *
     * @throws \LogicException
     */
    public function setAttribute($key, $value)
    {
        if (! property_exists($this, 'immutables')) {
            throw new LogicException('Class must define an $immutables array property.');
        }

        // First we will check for the presence of the attribute in the immutables
        // array property if found then we'll check if a value has already been
        // set in the model attributes. If both conditions are met just bail.
        if (in_array($key, $this->immutables) && isset($this->attributes[$key])) {
            return $this;
        }

        return parent::setAttribute($key, $value);
    }
}

ImmutablesTest.php

<?php

namespace Tests\Unit\Models\Traits;

use App\Models\Traits\Immutables;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Support\Carbon;
use LogicException;
use Tests\TestCase;

class ImmutablesTest extends TestCase
{
    /** @test */
    public function it_throws_an_exception_if_immutables_property_is_missing()
    {
        $this->expectException(LogicException::class);

        $model = new class extends Model {
            use Immutables;
        };

        $model->year = '2018-01-01 00:00:00';
    }

    /** @test */
    public function it_does_not_allow_to_change_a_property_after_it_is_been_set()
    {
        $model = new class extends Model {
            use Immutables;
            protected $immutables = ['validated_at'];
        };
        $this->assertNull($model->validated_at);

        $model->validated_at = '2018-01-01 00:00:00';
        $this->assertEquals('2018-01-01 00:00:00', $model->validated_at);

        $model->validated_at = null;
        $this->assertNotNull($model->validated_at);
        $this->assertEquals('2018-01-01 00:00:00', $model->validated_at);
    }

    /** @test */
    public function it_works_well_with_attribute_mutators()
    {
        $model = new class extends Model {
            use Immutables;
            protected $immutables = ['validated_at'];

            public function setValidatedAtAttribute($value)
            {
                $this->attributes['validated_at'] = Carbon::parse($value)->addYear();
            }
        };
        $this->assertNull($model->validated_at);

        $model->validated_at = '2018-01-01 00:00:00';
        $this->assertEquals('2019-01-01 00:00:00', $model->validated_at);

        $model->validated_at = null;
        $this->assertEquals('2019-01-01 00:00:00', $model->validated_at);
    }
}

Subscription.php

<?php

namespace App\Models;

class Subscription extends Model
{
    use Traits\Immutables;
  
    /**
     * The attributes that should not be mutated once they are set.
     *
     * @var array
     */
    protected $immutables = ['started_at'];
}