Swap values using PHP

<?php
if(isset($_POST['submit']))
{
    $a = $_POST['a'];
    $b = $_POST['b'];
    $temp = $a;
    $a = $b;
    $b = $temp;
}
?>

<form method="post">
<h1>SWAP VALUES USING PHP</h1>
    Value A: <input type="text" name="a"><br>
    Value B: <input type="text" name="b"><br>
    <input type="submit" name="submit" value="Swap">
</form>

<?php
if(isset($_POST['submit']))
{
    echo "After swapping: <br>";
    echo "Value A: ".$a."<br>";
    echo "Value B: ".$b."<br>";
}
?>