Sorry, ecco a te

Codice PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
    
<script type="text/javascript">
function selectAll() {
        var x=document.getElementById("xyz");         
          for (i=0;i<x.length;i++){
         x.options[i].selected=true;
        }
   
    }
</script>    
</head>
<body>
    [url="#"]All[/url]
    
    
    <select multiple="multiple" id="xyz" name="xyz">
        <option>option</option>
        <option>option</option>
        <option>option</option>
        <option>option</option>
        <option>option</option>
    </select>
</body>
</html>