Need Help With function str replace

[lcp03o]

Hi, Please help me with the function str replace command. I have searched the web and I can not work out what to do. I am using the following code

<a href="$thread[threadid]-$thread[threadtitle].html">

This basically puts a titles in the url, But I want to replace the spaces with a - can this be done with function str replace ?? If so what code do I have to add??

Thanks

[Installer]

$str = $thread['threadid'] . '-' . $thread['threadtitle'];
$str = str_replace(' ', '-', $str);
echo '<a href="' . $str . '.html">';