Database.php

<?php // lib/Database.php

namespace lib;

class Database
{
    
}

test.php

<?php // test.php

require __DIR__.'/lib/Tools.php';

$tools = new Tools();

Tools.php

<?php // lib/Tools.php

require __DIR__.'/Database.php';

namespace lib;

class Tools
{
    function something()
    {
        $db = new Database();
    }
}