如何在POST范围内声明一个php变量

I have this code and it works.

echo "<input type='submit' name='liga' value='Liga'>";
if (isset($_POST['liga'])) {
    unset($_POST['liga']);
    liga();
?>

But I need change the name LIGA to the php variable $on. I tried this but don't work.How can I insert the variable in this code?

$on=1;
 echo "<input type='submit' name='$on' value='Liga'>";

if (isset($_POST['$on'])) {
    unset($_POST['$on']);
    liga();
}
?>

The problem here is that variables inside single quotes don't get interpolated whereas variables inside double quotes do.

echo "<input type='submit' name='$on' value='Liga'>";

becomes

<input type='submit' name='1' value='Liga'>

But $_POST['$on'] stays the same. To solve this, use $_POST[$on] (or the equivalent $_POST["$on"]).