mysql_real_escape_string function not working

Question

I have been trying to convert my code from MySQL to MySQLi. I am trying to protect from sql injection. I have learned not to use pre_replace. I have been trying the different options as specified in my code below. The only other option that seems to work is the line of code that has mysql_escape_string below. I have tried mysql_real_escape_string and $db->real_escape_string as specified below. However, this causes the website to stop functioning all together. I am not receiving an error message though. I was wondering why the first line of code for $FName works and the following two lines of code won't work. I have spent about 2 hours trying everything I could think of. Sorry if this question is basic but I can't find the answer. Any help would be appreciated.

<?php require "connect.php"; ?>
<?php
   if(isset($_POST['Register'])) {
    session_start();
    $FName = mysql_escape_string($_POST['FirstName']);
    $LName = mysql_real_escape_string($_POST['LastName']);
    $Email = $db->real_escape_string($_POST['Email']);
    $UName = preg_replace('#[^A-Za-z0-9]#i', '', $_POST ["UserName"]);          

$PW = preg_replace('#[^A-Za-z0-9]#i', '', $_POST ["Password"]); 
$sql = $con->query("INSERT INTO BD (FirstName, LastName, Email, UserName, Password) Values('{$FName}', '{$LName}', '{$Email}', '{$UName}','{$PW}')");

   header('Location: login.php');
}

?>

Answer 1

You stated in comments that this is your connection:

$con =new mysqli ("local host", "name", "PW", "users")

yet you're using $db as the variable for $Email. That should be $con.

Plus, local host should be in one word, localhost.

$con =new mysqli ("localhost", "name", "PW", "users")

while checking for errors for it:

$con =new mysqli ("localhost", "name", "PW", "users");

if ($con->connect_error) {
    die('Connect Error (' . $con->connect_errno . ') '
            . $con->connect_error);
}

• Ref: http://php.net/manual/en/mysqli.construct.php

Then these will never work:

$FName = mysql_escape_string($_POST['FirstName']);
$LName = mysql_real_escape_string($_POST['LastName']);

as you are mixing MySQL APIs. Those different APIs/functions do not intermix with each other. You need to use the same from connection to query.

Including:

$Email = $db->real_escape_string($_POST['Email']);

Therefore, this whole block:

$FName = mysql_escape_string($_POST['FirstName']);
$LName = mysql_real_escape_string($_POST['LastName']);
$Email = $db->real_escape_string($_POST['Email']);

needs to be changed to:

$FName = $con->real_escape_string($_POST['FirstName']);
$LName = $con->real_escape_string($_POST['LastName']);
$Email = $con->real_escape_string($_POST['Email']);

---

Add error reporting to the top of your file(s) which will help find errors.

<?php 
error_reporting(E_ALL);
ini_set('display_errors', 1);

// rest of your code

Sidenote: Error reporting should only be done in staging, and never production.