Kodlar bu şekilde çalışıyor .
$wpdb bunu global $wpdb; olarak tanımlaman gerekebilir belki mysql den data geliyor mu kontrol et.

           <?php
       
        /**
        Template Name: Giris
        */
        session_start();
        ob_start();   
    if ($_POST)
{
        $mail = $_POST['mail'];
        $pass = $_POST['pass'];
        define('site_uniqe', 's3y3m3kHsn');
       
            $_SESSION['uye_online'] = true;
            $_SESSION['uye_adi'] = $mail;
            $_SESSION['uye_soyadi'] = $pass;
            header('Location: [URL]http://www.site.net'[/URL]);
       
}
else
{
?>
<form action="<?php echo $_SERVER["PHP_SELF"];?>/giris" method="POST" class="form-signin">      
            <h3 class="form-signin-heading">Hosgeldiniz! Lütfen Giris Yapin</h3>
              <hr class="colorgraph"><br />   
            <input type="text" class="form-control" name="mail" placeholder="E-Posta" required="" autofocus="" />
            <input type="password" class="form-control" name="pass" placeholder="Sifre" required=""/>
            <input type="submit" class="btn btn-lg btn-primary btn-block"  name="Submit" value="Giris Yap" />
            <div style="color:#f57f2f;margin-top:5px;text-align:center;font-size:16px;"><a href=""><b>Sifremi</b> Unuttum</a></div>
</form>   
<?php
}
  ?>
         
          <?php
        ob_end_flush();
        ?>