Código PHP:
<script language="JavaScript">
function showHide(o)
{
var form = document.frm;
if(!o.checked){
form.drop.style.visibility = 'hidden'
}else{
form.drop.style.visibility = 'visible'
}
return true;
}
</script>
<form action="" method="post" name="frm">
<select name="drop" size="1">
<option value="" selected>[SELECT]</option>
<option value="Uno">Uno</option>
<option value="Dos">Dos</option>
</select><br>
<input type="checkbox" name="C1" value="ON" checked onClick="return showHide(this)">
<font face="Arial" size="1">Mostrar / Ocultar</font>
</form>
alguien me puede ayudar...