title: EasyRDF - SPARQL querying DBpedia - "Undefined namespace prefix" error
body: <p>It's my first question. I am making a simple program to query DBpedia. I use PHP API + <a href="http://www.easyrdf.org" rel="nofollow noreferrer">EasyRdf RDF Library for PHP</a>.</p>
  
  <p>The SPARQL query is correct; it's working fine on <a href="http://dbpedia.org/snorql" rel="nofollow noreferrer">http://dbpedia.org/snorql</a>. I can use query for API example; it is also correct. I have <code>prefix</code> with <code>dbo</code>, <code>foaf</code>, <code>rdfs</code>...</p>
  
  <p>But when I use this query with this condidion <em><code>?person dbo:birthPlace :Berlin .</code></em>, I have this error:</p>
  
  <blockquote>
    <p>Fatal error:</p>
    
    <p>Uncaught exception 'EasyRdf_Exception' with message 'HTTP request for SPARQL query failed:</p>
    
    <p><strong>Virtuoso 37000</strong> Error SP030: SPARQL compiler, line 4: Undefined namespace prefix at '' before '.' SPARQL query: define sql:big-data-const 0 PREFIX foaf:  PREFIX rdfs:  PREFIX dbo:  SELECT ?name ?person WHERE { ?person a dbo:MusicalArtist . ?person dbo:birthPlace :Berlin . ?person foaf:name ?name . ?person rdfs:comment ?description . } ORDER BY ?name' in D:\xampp\htdocs\HelloComposer\lib\EasyRdf\Sparql\Client.php:290 Stack trace: #0 D:\xampp\htdocs\HelloComposer\lib\EasyRdf\Sparql\Client.php(120): EasyRdf_Sparql_Client->request('query', 'SELECT ?name ?p...') #1 D:\xampp\htdocs\dbpedia\index.php(43): EasyRdf_Sparql_Client->query('SELECT ?name ?p...') #2 {main} thrown in D:\xampp\htdocs\HelloComposer\lib\EasyRdf\Sparql\Client.php on line 290</p>
  </blockquote>
  
  <p>My PHP code --</p>
  
  <pre><code>&lt;?php
  
  
  
  require_once('D:\xampp\htdocs\HelloComposer\lib\EasyRdf.php');
  require_once ('D:\xampp\htdocs\HelloComposer\lib\html_tag_helpers.php');
  
  //PREFIX
  
  EasyRdf_Namespace::set('category', 'http://dbpedia.org/resource/Category:');
  EasyRdf_Namespace::set('dbpedia', 'http://dbpedia.org/resource/');
  EasyRdf_Namespace::set('dbo', 'http://dbpedia.org/ontology/');
  EasyRdf_Namespace::set('dbp', 'http://dbpedia.org/property/');
  EasyRdf_Namespace::set('foaf', 'http://xmlns.com/foaf/0.1');
  EasyRdf_Namespace::set('rdfs', 'http://www.w3.org/2000/01/rdf-schema#');
  
  
  
  
  $sparql = new EasyRdf_Sparql_Client('http://dbpedia.org/sparql');
  ?&gt;
  
  &lt;html&gt;
  &lt;head&gt;
    &lt;title&gt;EasyRdf Basic Sparql Example&lt;/title&gt;
    &lt;meta http-equiv="content-type" content="text/html; charset=utf-8" /&gt;
  &lt;/head&gt;
  &lt;body&gt;
  &lt;h1&gt;EasyRdf Basic Sparql Example&lt;/h1&gt;
  
  &lt;h2&gt;List of artists&lt;/h2&gt;
  &lt;ul&gt;
  &lt;?php
      $result = $sparql-&gt;query(
          'SELECT ?person ?name ?description WHERE {'.
          '  ?person a dbo:MusicalArtist .'.
          '  ?person dbo:birthPlace :Berlin .'.
          '  ?person foaf:name ?name .'.
          '  ?person rdfs:comment ?description . '.
          '  FILTER (LANG(?description) = "en") .'.
          '} ORDER BY ?name'
      );
      foreach ($result as $row) {
          echo "&lt;li&gt;".link_to($row-&gt;name, $row-&gt;person)."&lt;/li&gt;\n";
      }
  ?&gt;
  &lt;/ul&gt;
  &lt;p&gt;Total number of artists: &lt;?= $result-&gt;numRows() ?&gt;&lt;/p&gt;
  
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>Please... help me.</p>