为什么XPath找不到标签？

I have xml file and PHP script o process this XML. Why script can't find tags? Why xpath is failing with warnings:

PHP Warning:  DOMXPath::query(): Invalid predicate in test.php on line 7
PHP Warning:  DOMXPath::query(): Invalid expression in test.php on line 7
PHP Warning:  Invalid argument supplied for foreach() in test.php on line 9

doc.xml

<?xml version="1.0" encoding="UTF-8"?>
<rootTag>
    <div id="tag-id"></div>
    <div id="tag-id"></div>
    <div id="tag-id"></div>
    <div id="tag-id"></div>
    <div id="tag-id"></div>
</rootTag>

test.php

<?php

$doc = new DOMDocument();
$doc->load('./doc.xml');

$xpath = new DOMXPath($doc);
$tags = $xpath->query('//div[@id="tag-id"');

foreach ($tags as $node) {
    print($node->nodeName);
}

?>

Well, you query is almost correct. You have just forgotten putting last closing "]" char.

It ought to be:

$tags = $xpath->query('//div[@id="tag-id"]');

instead:

$tags = $xpath->query('//div[@id="tag-id"');