PHP/SQL Select Statement Does Not Work
<form action="checknumbers.php" method="post">

<table width="50%" border="0">
  <tr>
    <td><label for="ni">National Insurance Number</label>
</td>
    <td><span id="sprytextfield1">
    <input type="text" name="ni" id="ni" />
    <span class="textfieldRequiredMsg">*</span><span class="textfieldMaxCharsMsg">*</span></span></td>
  </tr>
  <tr>
    <td><br /><label for="ref">Reference Number</label>
</td>
    <td><br /><span id="sprytextfield2">
    <input type="text" name="ref" id="ref" />
    <span class="textfieldRequiredMsg">*</span></span></td>
  </tr>
  <tr>
    <td>&nbsp;</td>
    <td><br /><br /><input name="" type="submit" value="Continue" /></td>
  </tr>
</table>

</form>

<?php
$host="localhost"; // Host name
$username="**"; // Mysql username
$password="**"; // Mysql password
$db_name="***"; // Database name
$tbl_name="public"; // Table name

// Connect to server and select databse.
mysql_connect("$host", "$username", "$password")or die("cannot connect");
mysql_select_db("$db_name")or die("cannot select DB");


$ni=$_POST['ni'];
$ref=$_POST['ref'];

// To protect MySQL injection (more detail about MySQL injection)
$ni = stripslashes($username);
$ref = stripslashes($password);
$ni = mysql_real_escape_string($username);
$ref = mysql_real_escape_string($password);

$sql="SELECT * FROM $tbl_name WHERE ni ='$ni' AND ref='$ref' AND active = 'not_activated'";
$result=mysql_query($sql);

// Mysql_num_row is counting table row
$count=mysql_num_rows($result);
// If result matched $myusername and $mypassword, table row must be 1 row

if($count==1){

header("location:securityquestion.php");
}

else {

    echo '<hr><h4>Your National Insurance Number Or Reference Number Does Not Match</h4><hr><a href="register.html">Please try Again</a>';
}
?>

$ni=$_POST['ni'];
      $ref=$_POST['ref'];    // To protect MySQL injection (more detail about MySQL injection)
      $ni = stripslashes($username);
      $ref = stripslashes($password);
      $ni = mysql_real_escape_string($username);
      $ref = mysql_real_escape_string($password);

$ni= mysql_real_escape_string( stripslashes( $_POST['ni'] ) );
 $ref= mysql_real_escape_string( stripslashes( $_POST['ref'] ) );

$ni = stripslashes($username);
$ref = stripslashes($password);

$ni = stripslashes($ni);
$ref = stripslashes($ref);