Random errors in artisan

Question

Whenever I launch php artisan I get random errors that change after a while even if I don't modify files. Also those errors make no sense to me. For example:

PHP Parse error:  syntax error, unexpected '++' (T_INC), expecting '(' in /.../app/Providers/AuthServiceProvider.php on line 32

PHP Parse error:  syntax error, unexpected '?' in /.../app/Providers/AuthServiceProvider.php on line 32

PHP Parse error:  syntax error, unexpected '' in /.../app/Providers/AuthServiceProvider.php on line 32

AuthServiceProvider.php:

<?php

namespace App\Providers;

use Illuminate\Contracts\Auth\Access\Gate as GateContract;
use Illuminate\Foundation\Support\Providers\AuthServiceProvider as ServiceProvider;

class AuthServiceProvider extends ServiceProvider
{
    /**
     * The policy mappings for the application.
     *
     * @var array
     */
    protected $policies = [
        'App\Model' => 'App\Policies\ModelPolicy',
    ];

    /**
     * Register any application authentication / authorization services.
     *
     * @param  \Illuminate\Contracts\Auth\Access\Gate  $gate
     * @return void
     */
    public function boot(GateContract $gate)
    {
        $this->registerPolicies($gate);

        //
    }
}

Line 32 is a newline at the end of file.

Sometimes artisan works successfully (again without me changing any files). What may be happening there?

PHP version 5.6, Laravel 5.2.45