To define a mutator, define a setFooAttribute method on your model where Foo
  is the "studly" cased name of the column you wish to access. So, again,
lets define a mutator for the first_name attribute. This mutator will
be automatically called when we attempt to set the value of the first_name
attribute on the model:

class User extends Model
{
    public function setFirstNameAttribute($value)
    {
        $this->attributes['first_name'] = strtolower($value);
    }
}