function adjs(url){
alert("Despues del evento onChange "+url);
oldsc=document.getElementById("old_sc");
if(oldsc)
document.getElementsByTagName('body')[0].removeChild(oldsc);
sc=document.createElement('script');
alert("Elemento Script Creado "+sc);
//sc.id="old_sc";
sc.src=url+'&'+Math.random();
document.getElementsByTagName('body')[0].appendChild(sc);
alert("Ponen "+sc.src);
}

<?php
if(isset($_GET['p'])){
switch($_GET['sel']){
case '1':
$ret=array('Final del Juego','Rayuela','El Señor de loas Anillos');
break;
case '2':
$ret=array('rock','new age');
break;
case '3':
$ret=array('español','php','javascript');
break;
default:
echo 'document.getElementById("pp").innerHTML="<select name=\"dos\" id=\"dos\"></select>";';
exit;
}
$html='<select name=\"dos\" id=\"dos\">';
foreach($ret as $v)
$html.='<option value=\"'.$v.'\">'.$v.'</option>';
$html.='</select>';
echo 'document.getElementById("pp").innerHTML="'.$html.'";';
exit;
}
?>