$_POST problem

    <?php
            SESSION_START();
            $db = new PDO('sqlite:./it_stock.sqlite');

            if($_GET["create"] == "stepOne"){
                ?>
                <!-- begin to create new entry -->
                <form action="?id=<?php echo $_GET["id"]; ?>&amp;create=stepTwo" method="post">
                    <button name="action_bNew">Neu</button>
                </form>
                <!---->
                <?php

                //##########table (borrowed)##########//
                echo "<table class=\"sortable\">";

                //head
                echo  "<thead><tr>";
                echo "<th>Anzahl</th>";
                echo "<th>Name</th>";
                echo "<th>Von</th>";
                echo "<th>Bis</th>";
                echo "<th>Status</th>";
                echo "</tr></thead>";

                //body
                echo "<tbody>";
                foreach($db->query("SELECT * FROM db_borrowed WHERE id_db_inventory='".$_SESSION["id"]."'") as $row) {
                    echo "<tr>";
                    echo "<td>".$row["amount"]."</td>";
                    echo "<td>".$row["name"]."</td>";
                    echo "<td>".$row["from_date"]."</td>";
                    echo "<td>".$row["to_date"]."</td>";
                    echo "<td>".$row["status"]."</td>";
                    echo "</tr>";
                }
                echo "</tbody>";

                //foot
                echo "<tfoot>";
                //no content
                echo "</tfoot>";

                echo "</table>";
                //##########table (borrowed)##########//
                ?>
                <br>
                <br>
                <?php
            }
            if($_GET["create"] == "stepTwo"){
                ?>
                <!-- create a new entry -->
                <form action="?id=<?php echo $_GET["id"]; ?>&amp;create=stepThree" method="post">
                    <b>Name &nbsp;</b>
                    <input type="text" size="20" name="name_b">
                    <br>
                    <b>Anzahl &nbsp;</b>
                    <input type="text" size="5" name="amount_b">
                    <br>
                    <b>Anmerkung &nbsp;</b>
                    <input type="text" size="25" name="notice_b">
                    <br>
                    <br>
                    <button name="action_bCreate">Erstellen</button>
                </form>
                <!---->

                <!-- one page back -->
                <br>
                <br>
                <form action="?id=<?php echo $_GET["id"]; ?>&amp;create=stepOne" method="post">
                    <button name="back_bOne">Zur&uuml;ck</button>
                </form>
                <!---->
                <?php

            }

$name_ab = $_POST["name_b"];

            SESSION_START();
            if($_GET["create"] == "stepFour"){
                echo "$name_ab";
                if($_POST["action_bConfirm"] == "submit"){
                    // $stmt = $db->prepare("INSERT INTO db_inventory(id_db_inventory, name, from_date, to_date, amount) VALUES(?,?,?,?,?)");
                    // $stmt->bindParam(1, $a);
                    // $stmt->bindParam(2, $b);
                    // $stmt->bindParam(3, $c);
                    // $stmt->bindParam(4, $d);
                    // $stmt->bindParam(5, $e);
                    // $a = $_SESSION["amount_c1"];
                    // $b = $_SESSION["object_c1"];
                    // $stmt->execute();


                    echo "Das Objekt wurde ausgeliehen.";
                }
                if($_POST["action_bConfirm"] == "decline"){
                    echo "Ausleih-Vorgang abgebrochen.";
                }
                ?>
                <?php
            }
            if($_GET["create"] == "stepThree"){
                echo "$name_ab";
                ?>
                <form action="?id=<?php echo $_GET["id"]; ?>&amp;create=stepFour" method="post">
                    <b>Wirklich erstellen?</b>
                    <br>
                    <button name="action_bConfirm" value="submit">Erstellen</button>
                    <button name="action_bConfirm" value="decline">Abbrechen</button>
                </form>

                <!-- one page back -->
                <br>
                <br>
                <form action="?id=<?php echo $_GET["id"]; ?>&amp;create=stepTwo" method="post">
                    <button name="back_bOne">Zur&uuml;ck</button>
                </form>
                <!---->
                <?php
            }
            $db = NULL;
            ?>

            <!-- back to main menu -->
            <form action="index.php" method="post">
                <?php if($_GET["create"] == "stepThree"){
                    die();
                } ?>
                <button name="back_bMenu">Zur&uuml;ck zum Hauptmen&uuml;</button>
            </form>
            <!---->