Arbitrarily deeply nested parent child relationships with eloquent

Foo.php

<?php

use Illuminate\Eloquent\Model;

class Foo extends Model
{
    protected $attributes = [
        'parent_id' => null,
    ];
    
    public function children()
    {
        return $this->hasMany(static::class, 'parent_id');
    }
    
    public function allChildren()
    {
        return $this->children()->with('allChildren');
    }
}