I did install apache2-mod_php5-5.0.4-9.6

when I test this php code
 <?php
 $dom = new domDocument;
 $dom->loadXML('<books><book><title>blah</title></book></books>');
 if (!$dom) {
    echo 'Error while parsing the document';
    exit;
 }

 $s = simplexml_import_dom($dom);

 echo $s->book[0]->title; // blah
 ?> 

The Server respond with the next message:
Fatal error:  Call to undefined function simplexml_import_dom() in /srv/www/htdocs/simplexml.php on line 9

In the phpinfo() the simplexml is enabled.

The other php5 simplexml functions examples of PHP manual run well.
http://www.php.net/manual/es/ref.simplexml.php

The same code rules with the CLI version php5-5.0.4-9.6