<?php
require __DIR__.'/vendor/autoload.php';
use Monolog\Logger as Log;

$log = new Log('name');
$log->pushHandler(new Monolog\Handler\StreamHandler('app.log',Monolog\Logger::WARNING));
$log->warning('Foo 2.0');

<?php

namespace acme;

class Foo 
{
    
    function test()
    {
        echo "123456";
    }
}

{
    "require": {
        "monolog/monolog": "^2.1"
    },
    "autoload": {
        "psr-4":{"acme\\":"vendor/acme/"}
    }
}

<?php
require __DIR__.'/vendor/autoload.php';

use acme\Foo;

$d = new Foo;

$d->test();