Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>test</title>
<script>
function llenar(donde,que,estado,nom){
e=document.getElementById(donde);
op=[];
while(e.childNodes[0])e.removeChild(e.childNodes[0]);
if(!estado)return;
s=document.createElement('select');
s.name=nom;
for(j in que){
op[j]=document.createElement('option');
op[j].value=que[j]
document.all?op[j].label=que[j]:op[j].text=que[j];
s.appendChild(op[j])
}
e.appendChild(s);
}
</script>
</head>
<body>
<form id="form1" name="form1" method="post" action="<?php echo basename($_SERVER['PHP_SELF']) ?>">
<input type="checkbox" name="checkbox" value="checkbox" onclick="llenar('uno',['a','b','c'],this.checked,'sel1')" />
<span id="uno"></span><br />
<input type="checkbox" name="checkbox2" value="checkbox" onclick="llenar('dos',['uno','dos','tres'],this.checked,'sel2')" />
<span id="dos"></span>
<br />
<input type="checkbox" name="checkbox3" value="checkbox" onclick="llenar('tres',['rojo','azul','verde'],this.checked,'sel3')" />
<span id="tres"></span>
<input type="submit" name="Submit" value="Enviar" />
</form>
<?php if(isset($_POST['Submit']) && !empty($_POST['Submit'])){
echo '<pre>';
print_r($_POST);
echo '</pre>';
}
?>
</body>
</html>