Why eloquent model relationships do not use parenthesis and how do they work?

Question

I kept searching the web for an hour but couldn't figure this out. If we look at the eloquent relationships documentation: https://laravel.com/docs/5.2/eloquent-relationships

The example User model:

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class User extends Model
{
    /**
     * Get the phone record associated with the user.
     */
    public function phone()
    {
        return $this->hasOne('App\Phone');
    }
}

Just below it, how to access the phone number of a user with id=1:

$phone = User::find(1)->phone;

Why is it phone and not phone() and what is the difference?

Also how does it work? If I try to call an object->name without parenthesis in my code PHP thinks I am looking for a class variable named name?

Some extra information:

It looks like phone is returning object(App\Models\Phone) and phone() is returning object(Illuminate\Database\Eloquent\Relations\HasOne)

If I run the code below:

User::find(1)->phone->count()

Framework executes following SQL statements:

select * from `phone` where `phone`.`user_id` = '1' and `phone`.`user_id` is not null limit 1
select count(*) as aggregate from `phone`

If I run the code below:

User::find(1)->phone()->count()

Framework executes following SQL statement:

select count(*) as aggregate from `phone` where `phone`.`user_id` = '1' and `phone`.`user_id` is not null

Answer 1

One way of thinking about it is that the public function phone() function defines the relationship, so using $obj->phone() would get you the Eloquent relationship itself (not the results of that relationship) which you could then modify with various query builder elements if you wanted.

Leaving out the brackets is the Eloquent shorthand for adding ->get() or ->first() at the end of the expression (Eloquent knows which to use based on if it's a hasOne, hasMany, etc. relationship, as defined in the function), which returns an Eloquent collection.

So, $obj->phone is the same as $obj->phone()->first().