Modify datastream within PHP using tuque

gistfile1.phtml

<?php
module_load_include('inc', 'islandora_basic_image', 'includes/derivatives');

$pid = "islandora:13";
$fedora_object = islandora_object_load($pid);

if (!$fedora_object) {
  drupal_set_message("Fedora Object isn't in the repo!");
} 
if (!isset($fedora_object["OBJ"])) {
  $obj_datastream = $fedora_object->constructDatastream('OBJ', 'M');
}
else {
  $obj_datastream = $fedora_object["OBJ"];
}
//dpr($obj_datastream);
$obj_datastream->label = 'OBJ';
$obj_datastream->mimetype = 'image/png';
$obj_datastream->setContentFromUrl('http://ratatosk.net/assets/screenshots/debugging-drupal/devel-dpr.png');
$fedora_object->ingestDatastream($obj_datastream);
islandora_basic_image_create_all_derivatives($fedora_object);
?>