Add auth before requesting node value.
<?php
$zookeeper = new Zookeeper('localhost:2181');
$path = '/path/to/node';
$value = 'nodevalue';
$zookeeper->set($path, $value);
$zookeeper->addAuth('digest', 'user0:passwd0');
$r = $zookeeper->get($path);
if ($r)
echo $r;
else
echo 'ERR';
?>
The above example will output: