要求：PHP >= 5.5

<?php

class User {

}

echo User::class; // return User

<?php
namespace App\Models;

class User {

}
echo User::class; // return App\Models\User

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Phone extends Model
{
    /**
     * Get the user that owns the phone.
     */
    public function user()
    {
        return $this->belongsTo('App\Models\User');
        //return $this->belongsTo(User::class);
    }
}