Tambien puedes hacer esto:
Código PHP:
$html = '
<form name="jumpy">
<select name="example" size="1" onChange="gone()">
<option value="http://www.yahoo.com" selected>Yahoo.com</option>
<option value="http://www.google.com">Google</option>
<option value="http://www.lycos.com">Lycos</option>
<option value="http://www.AltaVista.com">AltaVista</option>
</select>
<input type="button" name="test" value="Go!" onClick="gone()">
</form>
<script language="javascript">
<!--//
var displaymode=1
var iframecode=\'<iframe id="external" style="width:95%;height:400px" src="http://www.yahoo.com"></iframe>\'
if (displaymode==1)
document.write(iframecode)
function gone(){
var selectedurl = document.jumpy.example.options[document.jumpy.example.selectedIndex].value
if (document.getElementById && displaymode==1)
document.getElementById("external").src = selectedurl
else
if (document.all&&displaymode==1)
document.all.external.src=selectedurl
else{
if (!window.win2||win2.closed)
win2=window.open(selectedurl)
else{
win2.location=selectedurl
win2.focus()
}
}
}
//-->
</script>';
echo $html;
Suerte!