¿Oh, o sea que si se selecciona cualquier día de la semana (o cualquiera) no esté seleccionado No definió? En todo caso, eso es javascript. Pero algo sencillo con javascript es así
Código PHP:
<script type="text/javascript">
function addListener(element, type, expression, bubbling)
{
bubbling = bubbling || false;
if(element.addEventListener) { // Standard
element.addEventListener(type, expression, bubbling);
return true;
} else if(element.attachEvent) { // IE
element.attachEvent('on' + type, expression);
return true;
} else return false;
}
var obj = document.getElementsByName('week');
function allChecksEmpty(){
for(var i in obj){
if(i > 0 && obj[i].checked){
return false;
}
}
return true;
}
function fixChecked(){
for(var i in obj){
addListener(obj[i], 'click', function(){
obj[0].checked = false;
if(allChecksEmpty()){
obj[0].checked = true;
}
});
}
}
window.onload = fixChecked;
</script>
<input type="checkbox" name="week" value="nd" checked="checked" /> ND<br />
<input type="checkbox" name="week" value="l" /> L<br />
<input type="checkbox" name="week" value="m" /> M<br />
<input type="checkbox" name="week" value="w" /> W<br />
<input type="checkbox" name="week" value="j" /> J<br />
<input type="checkbox" name="week" value="v" /> V<br />
<input type="checkbox" name="week" value="s" /> S<br />
<input type="checkbox" name="week" value="d" /> D