persistent session variable after being unset then page redirected

let me start directly with the code where the issue can be demonstrated and reproduced:

<?phpsession_start();// the outputif(isset($_SESSION["user"])){    assignSecret();    echo "welcome ".$_SESSION["user"]."<br>";    echo "your secret is ".$_SESSION["secret"];}else{    echo "no one is signed";}// sign out mechanismif(isset($_POST["signout"])){    // yes I want to unset only the user and keep all other session details    // it's a desired behaviour    unset($_SESSION["user"]);    header("Location: " . $_SERVER['PHP_SELF']);    exit();}// sign in mechanismif(isset($_POST["signin"])){    $_SESSION["user"]="world";    header("Location: " . $_SERVER['PHP_SELF']);    exit();}// random BSfunction assignSecret(){    unset($_SESSION);    // I actually reload session data from a database    // but for the sake of simplificy let's assume:    $_SESSION = ["user" => "world", "secret" => "xyz"];}?><!--All HTML stuff--><form method="post" action><input type="submit" name="signin" value="sign in"><input type="submit" name="signout" value="sign out"></form>

I can sign in fine, but when I click sign out, here is what I think the order of execution is (I used xdebug to confirm this):

1. $_SESSION["user"] is set and so assignSecret() is executed
2. $_POST["signout"] is also set and so $_SESSION["user"] gets unset
3. the script re-executes from the start because of header() function
4. at this point $_SESSION["user"] should be unset already, but it isn't, why ?

the expected result is for the signout to happen, no $_SESSION["user"] should be set any longer.but that doesn't happen.