Class variables NULL?

Question

I am working on a web back-end that will pull information into a form, and then when updated, will update the database with the new information. However, when I try to pull information previously stored in a class private variable, it throws me an error stating that the information is NULL. What am I doing wrong here?

<?php
class modify_racer
{
    private $mysqli, $racer_id, $firstname,
        $lastname, $banner, $bio;

    public function error($code)
    {
        switch($code)
        {
            case 1:
                echo '<p id="error"><b>Error:</b> Please fill out all fields!</p>';
                modify_racer::send_form($this->firstname, $this->lastname, $this->banner, $this->bio);
                break;
            case 2:
                echo '<p id="error"><b>Error:</b> Racer already exists!</p>';
                break;
            case 3:
                echo '<p id="error"><b>Error:</b> Could not connect to MySQLi: ' . mysqli_error();
                break;
        }
    }

    public function send_form($modify = 1)
    {
?>

<div id="form">
    <h3>Edit Racer:</h3>
    <form method="post" action="">
        <label for="firstname">First Name: </label>
        <input type="text" id="firstname" name="firstname"
            placeholder="Racer's First Name"
            value="<?php echo $this->firstname;?>" />
        <br />
        <label for="lastname">Last Name: </label>
        <input type="text" id="lastname" name="lastname"
            placeholder="Racer's Last Name"
            value="<?php echo $this->lastname;?>" />
        <br />
        <label for="banner">Banner Location: </label>
        <input type="text" id="banner" name="banner"
            placeholder="Racer's Banner Image Location:"
            value="<?php echo $this->banner;?>" />
        <br />
        <label for="bio">Racer's Bio Info: </label>
        <textarea rows="5" cols="50" id="bio" name="bio"
            placeholder="Racer Statistics / Biography"
            value=""><?php echo $this->bio;?></textarea>
        <input type="submit" id="submit" name="modify" value="submit" />
    </form>
</div>

<?php
    }

    public function get_racer($racerID)
    {
        $this->racer_id = $racerID;

        $this->mysqli = new mysqli(MYSQLI_HOST,MYSQLI_USER,MYSQLI_PASS,MYSQLI_DATABASE)
            or die(error(3));

        $racer_info = "SELECT * FROM ArtecRacers WHERE RacerID=?";
        $load_racer = $this->mysqli->prepare($racer_info);
        $load_racer->bind_param('s', $racerID);
        $load_racer->execute();
        $load_racer->bind_result($this->racerID, $this->firstname, $this->lastname, $this->banner, $this->bio);
        $load_racer->fetch();

        modify_racer::send_form();
    }

    public function list_racers()
    {
?>

<div id="form">
    <h3>Select Racer:</h3>
    <form method="post" action="">

        <?php
            $this->mysqli = new mysqli(MYSQLI_HOST,MYSQLI_USER,MYSQLI_PASS,MYSQLI_DATABASE)
                or die(error(3));
            $racer_list = "SELECT * FROM ArtecRacers";

            $get_racers = $this->mysqli->query($racer_list);

            while($list = $get_racers->fetch_array(MYSQLI_NUM))
            {
                echo '<input id="part" type="radio" name="editRacer" value="' . $list[0] . '"/>';
                echo '<label for="part">' . $list[1] . ' ' . $list[2] . '</label><br />';
            }
        ?>

        <input type="submit" name="selectRacer" id="submit" value="Select Racer" />
    </form>
</div>

<?php
    }

    function test2()
    {
        echo $this->firstname;
        echo $this->lastname;
        echo $this->racer_id;
    }
}

$start = new modify_racer();


if(!isset($_POST['selectRacer']))
    $start->list_racers();

if(isset($_POST['selectRacer']))
    $start->get_racer($_POST['editRacer']);

$start->test2();

?>

Everything in the code works except at $start->test2(); all of the information pulled from the function test2() is blank, and I am not sure why... Any insights?

EDIT:

I changed the code to reflect the following on the bottom, and test2() still outputs the variables as NULL:

if(!isset($_POST['editRacer']))
    $start->list_racers();
else
    $start->get_racers($_POST['editRacer']);

$start->test2();

Answer 1

If you leave your code alone, you're going to have to pass both selectRacer and editRacer parameters into the page. My guess is that you might only want to pass the one, though. In which case, you'll want to change

if(isset($_POST['selectRacer']))
    $start->get_racer($_POST['editRacer']);

into

if(isset($_POST['editRacer']))
    $start->get_racer($_POST['editRacer']);

Also, if you want to pass these values in through the URL bar, you need to check $_GET, not $_POST.

And finally, everywhere that you are making method calls by executing modify_racer::my_method_here(), you should change that to $this->my_method_here(). The former is a static method call, meaning it's not actually associated with your object, meaning it can't touch those variables. For it to be able to access and change the variables, you'll need to call it through $this.