After upgrading Laravel from 5.6 to 6.0, Call to undefined str_random() function not working

Likelihood Of Impact: High Laravel 6 Upgrade Guide

In Laravel 6 All str_ and array_ helpers have been moved to the new laravel/helpers Composer package and removed from the framework. If desired, you may update all calls to these helpers to use the Illuminate\Support\Str and Illuminate\Support\Arr classes. Alternatively, you can add the new laravel/helpers package to your application to continue using these helpers:

composer require laravel/helpers

If don't want to add Package then Used Str And Arr Classes.

For Example :

Str::random(12)

https://laravel.com/docs/master/helpers#method-str-random

Add the following string library.

use Illuminate\Support\Str;

now you can use it as below.

$filename = Str::random(40)

alternatively, install the following package.

composer require laravel/helpers