在Laravel项目的常规php文件中加载类

I have a laravel project and in it, I create a regular (.php) file that I am posting some data to. In that file, I want to use a class that I installed using composer in the Laravel project. When I post data to the page I am getting the error No such file in directory and require(): Failed opening required 'vendor/autoload.php. How can I resolve this my project is almost complete if I can get this working.

I've tried: require "class name" but the same error.

PHP file posting to:

<?php
    require "vendor/autoload.php";

    require 'vendor\lcobucci\jwt\src\Builder.php';
    require 'vendor\lcobucci\jwt\src\Signer.php';

    //I've tried using these require statements

    use \vendor\Lcobucci\JWT\Builder; 
    use \vendor\Lcobucci\JWT\Signer\Hmac\Sha256;

    someFunctiion(){
        // do stuff with function from class
    }
?>

I expect that when I post to this page, the function from the class I am 'using' will be called but instead, it's saying require() failed to open class.