title: Invalid XSD Schema in DOMDocument validation
body: <p>I have an XML document with such format:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;books-feed xmlns="{NS-URL}"&gt;
  &lt;generation-date&gt;{DATE}&lt;/generation-date&gt;
  &lt;book id="{BOOK-ID}"&gt;
      &lt;title&gt;{BOOK-TITLE}&lt;/title&gt;
      &lt;author&gt;{BOOK-AUTHOR}&lt;/author&gt;
      .......................................... ← other information tags [any]
  &lt;/book&gt;
  .............................................. ← other &lt;book&gt; elements
  &lt;/books-feed&gt;
  </code></pre>
  
  <p>And I'm trying to validate this file with this XSD-schema:</p>
  
  <pre><code>&lt;xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"&gt;
  &lt;xsd:element name="books-feed"&gt;
      &lt;xsd:complexType&gt;
          &lt;xsd:sequence&gt;
              &lt;xsd:element name="generation-date" type="xsd:string"/&gt;
              &lt;xsd:element name="book" maxOccurs="unbounded"&gt;
                  &lt;xsd:complexType&gt;
                      &lt;xsd:sequence&gt;
                          &lt;xsd:any maxOccurs="unbounded" processContents="lax"/&gt;
                      &lt;/xsd:sequence&gt;
                      &lt;xsd:attribute name="id" type="xsd:integer" use="required"/&gt;
                  &lt;/xsd:complexType&gt;
              &lt;/xsd:element&gt;
          &lt;/xsd:sequence&gt;
      &lt;/xsd:complexType&gt;
  &lt;/xsd:element&gt;
  &lt;/xsd:schema&gt;
  </code></pre>
  
  <p>So DOMDocument::schemaValidate() returns FALSE. Is this problem in namespaces (books-feed xmlns="{NS-URL}" not equals schema xmlns)? Or I need insert xsd:attribute for xmlns book-feed into schema (in will cause "Invalid schema" warning). Or what?</p>