My Multi-form Pages Works Great Until I Unset A Variable. Why?

I have been working on a multi-form page for a while. I need some help trying to understand why when I unset my variable that the multi-form pages ceases to work correctly. I place

Solution 1:

Values stored in the session must be discarded at the end of the script execution.

1. Destroy $_SESSION using unset($ _ SESSION) and
2. then destroy the session itself using session_destroy();

Solution 2:

I was able to clear the variable by using a windows popup once an if statement was fullfilled.

if ($Num_CTeams > 1 && $_SESSION["Pickleball"] !="") {
    echo'<script language="javascript">';
    echo'alert("Sorry, Captains can only captain up to two teams ONLY.")';
    echo'</script>';
    $_SESSION["formatPickleball"] = "";
} 

Solution 3:

<?php
session_start();

echo$_SESSION["formatPickleball"];
$_SESSION["Pickleball"] = "";

if (isset($_POST['format1']) && $_POST['format1'] && isset($_POST['member']) && $_POST['member']) {
    $_SESSION["formatPickleball"] = $_POST['member'];
    $_SESSION["Pickleball"] = "hide";
}

if ($_SESSION["formatPickleball"] == "Captain") {
    $regCAP = isset($_POST['submitCAP']) && $_POST['submitCAP'] ? "show": "";
    $secondCAP = isset($_POST['submit2']) && $_POST['submit2'] ? "show": "";
    $thirdCAP = isset($_POST['submit3']) && $_POST['submit3'] ? "show": "";
} 
if ($_SESSION["formatPickleball"] == "Player") {
    $regPlay =  isset($_POST['submitPlay']) && $_POST['submitPlay'] ? "show" : "";
}