call tracker-search from php

Question

I try to run gnome tracker-search from php. For example like this:

<?php
   $result = shell_exec("/usr/bin/tracker-search -l 5 --disable-snippets --disable-color test");
   echo "<pre>$result</pre>";
?>

But the output is empty. When I run /usr/bin/tracker-search --help in php, I get the help output. I also try to put tracker-search in the visudo file, but it have also no effect.

Have anybody an Idea what's happen here?