[PHP] php3 and php4

[Ozete C.Filho]

My friends, why this program no function in php4, just in php3?

Pessoal, esse algoritmo é conhecido como Dijkstra, ele calcula o melhor caminho entre 
pontos( num grafo, por exemplo), mas nao consigo rodar ele no php4, so funciona no 3. 
Alguem sabe me dizer se há algo errado?

<?
$neighbors[a] = array(b => 2, d => 3);
$neighbors[b] = array(a => 2, c => 1, e => 4);
$neighbors[c] = array(b => 1, f => 5);
$neighbors[d] = array(a => 3, e => 2);
$neighbors[e] = array(d => 2, b => 4, f => 1);
$neighbors[f] = array(c => 5, e => 1);

function dijkstra($neighbors, $start) {
  $closest = $start;
  while (isset($closest)) {
    $marked[$closest] = 1;
    reset($neighbors[$closest]);
    while(list($vertex, $distance) = each($neighbors[$closest])) {
      if ($marked[$vertex])
        continue;
      $dist = $paths[$closest][0] + $distance;
      if (!isset($paths[$vertex]) || ($dist < $paths[$vertex][0])) {
        $paths[$vertex] = $paths[$closest];
        $paths[$vertex][] = $closest;
        $paths[$vertex][0] = $dist;
      }
    }
    unset($closest);
    reset ($paths);
    while(list($vertex, $path) = each($paths)) {
      if ($marked[$vertex])
        continue;
      $distance = $path[0];
      if (($distance < $min) || !isset($closest)) {
        $min = $distance;
        $closest = $vertex;
      }
    }
  }
  return $paths;
}

$paths = dijkstra($neighbors, "$start");
while(list($vertex, $path) = each($paths))
  echo "$vertex: ", implode(", ", $path), "\n";

?>