Salva come pagina0.php

nella funzione che ti avevo postato c'era un
,
invece di
;
...

Codice PHP:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
    <title>Documento senza titolo</title>
    <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">

    <script language="javascript">
    function controllo(f)
    {
        var ckIndex = -1;
        var parametro = "";
    
        for (var i=0; i<3; i++)
        {
            var b = document.getElementById("ck"+i).checked;
            if (b)
            {
                ckIndex = i;
                parametro += i + " ";
            }
        }
        if (ckIndex>=0)
        {
            f.action="pagina0.php?value="+parametro;
        }
        else
        {
            f.action="pagina0.php?value=null";
        }
        
        alert ("action: "+f.action);
        f.submit();
    
    }
</script>

</head>
<body>

<?php
    echo "Valore letto: $_GET[value]";
?>    


<div align="center">
    <form name="form1" action="" method="post" onsubmit="return controllo(this)">
        <table width="200" border="1">
            <tr>
                <td><input type="checkbox" name="gruppo" id="ck0"></td>
                <td><input type="checkbox" name="gruppo" id="ck1"></td>
                <td><input type="checkbox" name="gruppo" id="ck2"></td>
            </tr>
            <tr>
                <td colspan="4">
                    <input type="submit" value="Invia">
                </td>
            </tr>
        </table>
    </form>
</div>
</body>
</html>