As of solr 4.0 the waitFlush parameter is removed.
Source
http://wiki.apache.org/solr/UpdateXmlMessages
$client->commit() throws an error
[SolrClientException]
Unsuccessful update request. Response Code 400. <?xml version="1.0" encoding="UTF-8"?>
<response>
<lst name="responseHeader">
<int name="status">400</int>
<int name="QTime">1</int>
</lst>
<lst name="error">
<str name="msg">Unknown commit parameter 'waitFlush'</str>
<int name="code">400</int>
</lst>
</response>