How to used php(using for loop) in switch case

How to used php(using  for loop) in switch case;

<input type="text" name="c"/>

<input type="submit" name="btn"/>

<select name="c">

<option value="green">Green</option>

</select>

<?php

if(isset($_POST['btn'])) {?>

<?php

 $color =$_POST['c'];

switch('$color') {

case 'red':

echo 'on';

break;

case 'green':

echo 'off';

break;

default:

echo 'none';

} 

}

?>