MyCode.php

<?php

class MyCode
{
    private function option1()
    {
        $this->notMyCode->doSomething(function ($value) {
            return strlen($value);
        });
    }

    private function option2()
    {
        $this->notMyCode->doSomething('strlen');
    }
}

NotMyCode.php

<?php

class NotMyCode
{
    public function doSomething(\Closure $callback)
    {
        // Nie mój kod, ale chcę z niego skorzystać, wymaga \Closure
    }
}