dependency.php

<?php
namespace bar;

class dependency
{

}

run.php

<?php
include __DIR__.'/superclass.php';
include __DIR__.'/subclass.php';
include __DIR__.'/dependency.php';

$a = new foo\subclass;
$a->test();

subclass.php

<?php
namespace foo;

use \bar\dependency;

class subclass extends superclass
{
    public function test()
    {
        $this->execute();
    }
}

superclass.php

<?php
namespace foo;

class superclass
{
    protected function execute()
    {
        $a = new dependency;
        
        var_dump($a);
    }
}