如何正确使用另一个文件中的一个文件的类？

I have one file DHMOFramework/lib/Core/Core.php with this code:

<?php

namespace DHMOFramework\lib\Core\Core;

/**
 * Main DHMOFramework class
 */

$registeredCommands = [];

class Core
{

  function registerCommand($command) {
    global $registeredCommands;
    $registeredCommands[$command] = [];
  }

  function registerSubCommand($commandname, $subcommand, $args, $helpmsg) {
    global $registeredCommands;
    $structure = array('command' => $commandname, 'subcommand' => $subcommand, 'args' => $args, "helpmsg" => $helpmsg);
    array_push($registeredCommands[$commandname], $structure);
    echo $registeredCommands;
  }

}

And I have another file test.php with this code:

<?php

require_once "DHMOFramework/lib/Core/Core.php";

$dhmo = new Core();

$dhmo->registerCommand("command");
$dhmo->registerSubCommand("command", "subcommand", ["arg1" => [true, string], "arg2" => [true, boolean]], "Usage: /command subcommand <arg1> <arg2>");

And I keep getting this error when I run the test.php file:

Fatal error: Class 'Core' not found in /home/ubuntu/workspace/test.php on line 5

Call Stack:
    0.0001     232464   1. {main}() /home/ubuntu/workspace/test.php:0

How ca I fix this?

Use full class name:

$dhmo = new DHMOFramework\lib\Core\Core\Core();

Or use a use statement before using the class:

use DHMOFramework\lib\Core\Core\Core;
$dhmo = new Core();

Check out Namespaces in Official PHP documentation