为什么我的代码在试图阻止Sql注入时不起作用

我是一名年轻的开发人员，试图在一个我最初没有自己编写代码的网站上阻止Sql注入。我读过很多关于注射及其背后的理论的文章，但我似乎无法理解。在我把头伸进监视器前，请帮帮我。谢谢我的显示器

这是代码：

<div id="main">
<div id="header"><? include('site_headergraphics.php'); ?></div>
<table width="800" height="100%" border="0" cellspacing="0" cellpadding="0" 
class="content">
  <tr>
    <td width="800" valign="top">
      <div style="padding:10px;">

  <? if($_SESSION[$_SESSION['SFIX'].'_owner_id'] == ''){ ?>
  <br><br>
        <form name="balance_login" method="post" action="<? if ($_GET['passthru']){    
?>?passthru=<?= $_GET['passthru'] ?><? } else { ?>?<? } ?>">
      <input name="action" type="hidden" value="login" />
      <table width="780" border="0" cellpadding="5" bgcolor="#FFFFFF">
        <tr>
          <td><table width="780" border="0" cellpadding="5" cellspacing="0" 
bgcolor="#F7C30F" class="body_text">
            <tr>
              <td colspan="3" align="left" class="rightmenu"><?= $passmessage ?><?= 
$specialmessage ?></td>
                </tr>
            <tr>
              <td colspan="2" align="left" valign="top" class="rightmenu">To sign on, 
enter your information below.</td>
                  <td width="412" rowspan="5" valign="top"><p>Welcome to Mediterranean 
Wellness!</p>
                  <p>Ready to join us?  You can <a 
href="index.php?section=payment">GET STARTED here. </a></p></td>
            </tr>
            <tr valign="top">
              <td width="121" align="right" class="rightmenu">Username:</td>
                  <td width="217"><input name="username" type="text" class="body_text" 
id="username" /></td>
                  <!--Added in the prevent SQL code injections-->
                  <?php /*?><?php unset($FindUser);       
                        if(isset($_POST['username']))
                        {
                            $_POST['username'] = 
trim($_POST['username']);
                            if(preg_match('/^[a-
zA-Z0-9^$.*+'[']{,}]{1,32}$/u', $_POST['username']))
                                $FindUser = 
$_POST['username'];
                        }
                         ?><?php */?>
                </tr>
            <tr valign="top">
              <td align="right" class="rightmenu">Password:</td>
              <td><input name="password" type="password" class="body_text" 
id="password" /></td>
                <?php /*?><?php unset($FindPass);
                        if(isset($_POST['password']))
                        {
                            $_POST['password'] = 
trim($_POST['password']);
                            if(preg_match('/^[a-
zA-Z0-9^$.*+'[']{,}]{1,32}$/u', $_POST['password']))
                                $$FindPass = 
$_POST['password'];
                        }
                         ?><?php */?>
                         <!--End of code added in the prevent SQL code injections-->
            </tr>
            <tr valign="top">
              <td>&nbsp;</td>
                  <td><input name="Submit" type="submit" class="body_text" 
value="Login" />
                   <?php 
                    function make_safe($variable) {
                        $variable = 
mysql_real_escape_string(trim($variable));
                        return $variable;
                    }
                    $username = make_safe($_POST['username']);
                    $password = make_safe($_POST['password']);
                    $check = mysql_query("SELECT Username, 
Password, UserLevel FROM Users WHERE Username = '".$username."' and Password = 
'".$password."'");
                    ?>
                  <br>
                  <br>
                  <a href="?section=forgot">Forgot your username or password?</a></td>
                </tr>
            <tr>
              <td>&nbsp;</td>
                  <td colspan="2">&nbsp;</td>
                </tr>
            </table></td>
          </tr>
        </table>
      </form>
    <? } else {  ?>
    <p class="body_text">You must <a href="?user_action=logout">logout</a> to continue 
to this page</p>
    <? } ?>
      </div></td>
    </tr>
</table>
</div>