bad code.php

<?php

class Controller
{
    public function actionRun()
    {
        $a = new Writer();
        $a->saveFile();

        return readfile('/var/www/file.pdf');
    }
}

Writer.php

<?php

class Writer
{
    public function saveFile()
    {
        file_put_contents('/var/www/file.pdf', '123');
    }
}