You don't have to explicitly declare a foreign key in the MySQL side but you have at least to create a post_id
column that will be used by Laravel as a foreign key.
Of course, you can name this column as you want and specify it in the declaration of the relation :
class Post extends Eloquent {
public function comments()
{
return $this->hasMany('Comment', 'post_primary_key');
}
}
You can also declare this column as a foreign key in PHPMyAdmin to improve robustness of your database but that's not Laravel business.