Basic Loader Interface with Delegate Lookup

loader.php

<?php

interface LoaderInterface {
    function load($resource);
    function supports($resource);
}

abstract class Loader implements LoaderInterface {
    private $delegate;
    private $loading = [];

    public function __construct(LoaderInterface $delegate=null) {
        if ($delegate === null) {
            $delegate = $this;
        }

        $this->delegate = $delegate;
    }

    protected function import($resource) {
        if (isset($this->loading[$resource])) {
            throw new RuntimeException("Cyclic reference for $resource");
        }

        $this->loading[$resource] = true;

        $result = $this->delegate->load($resource);

        unset($this->loading[$resource]);

        return $result;
    }
}

class LoaderComposite implements LoaderInterface {
    private $loaders;

    public function __construct(array $loaders=[]) {
        foreach ($loaders as $loader) {
            $this->addLoader($loader);
        }
    }

    public function addLoader(LoaderInterface $loader) {
        $this->loaders[] = $loader;
    }

    public function load($resource) {
        foreach ($this->loaders as $loader) {
            if ($loader->supports($resource)) {
                return $loader->load($resource);
            }
        }

        throw new RuntimeException(
            "Resource $resource has no registered loader"
        );
    }

    function supports($resource) {
        foreach ($this->loaders as $loader) {
            if ($loader->supports($resource)) {
                return true;
            }
        }

        return false;
    }
}