Topological sorting with directed cycle detection

topsort.php

<?php

$a = new Node('a');
$b = new Node('b');
$c = new Node('c');
$d = new Node('d');
$e = new Node('e');

$a->dependsOn($b);
$a->dependsOn($d);
$b->dependsOn($c);
$b->dependsOn($e);
$c->dependsOn($d);
$c->dependsOn($e);
$d->dependsOn($b);

$resolved = resolveDependency($a);
foreach ($resolved as $resolvedNode) {
    echo $resolvedNode->getName() . PHP_EOL;
}

function resolveDependency(Node $node, &$resolved = array(), &$seen = array()) {
    $seen[] = $node;
    foreach ($node->getEdges() as $edge) {
        if (!in_array($edge, $resolved)) {
            if (!in_array($node, $seen)) {
                resolveDependency($edge, $resolved);
            }
            else {
                throw new RuntimeException('Circular reference detected!');
            }
        }
    }
    $resolved[] = $node;
    return $resolved;
}

class Node
{
    private $name;
    private $edges = array();

    public function __construct($name)
    {
        $this->name = $name;
    }

    public function dependsOn(Node $node)
    {
        $this->edges[] = $node;
    }

    public function getName()
    {
        return $this->name;
    }

    public function getEdges()
    {
        return $this->edges;
    }
}