MySQL - Ayuda con Login PHP

<form role="form" name="registro" action="php/registro.php" method="post">

  <div class="form-group">

    <label for="username">Nombre de Usuario</label>

    <input type="text" class="form-control" id="username" name="username" placeholder="Nombre de Usuario">

  </div>

  <div class="form-group">

    <label for="fullname">Nombre Completo</label>

    <input type="text" class="form-control" id="fullname" name="fullname" placeholder="Nombre Completo">

  </div>

  <div class="form-group">

    <label for="email">Correo Electronico</label>

    <input type="email" class="form-control" id="email" name="email" placeholder="Correo Electronico">

  </div>

  <div class="form-group">

    <label for="password">Contrase&ntilde;a</label>

    <input type="password" class="form-control" id="password" name="password" placeholder="Contrase&ntilde;a">

  </div>

  <div class="form-group">

    <label for="confirm_password">Confirmar Contrase&ntilde;a</label>

    <input type="password" class="form-control" id="confirm_password" name="confirm_password" placeholder="Confirmar Contrase&ntilde;a">

  </div>

  <div class="form-group">

    <label for="Security">Security Word</label>

    <input type="text" class="form-control" id="Security" name="Security" placeholder="Security Word">

  </div>

<?php

if(!empty($_POST)){

	if(isset($_POST["username"]) &&isset($_POST["fullname"]) &&isset($_POST["email"]) &&isset($_POST["password"]) &&isset($_POST["confirm_password"])){

		if($_POST["username"]!=""&& $_POST["fullname"]!=""&&$_POST["email"]!=""&&$_POST["password"]!=""&&$_POST["password"]==$_POST["confirm_password"]){

			include "conexion.php";

			$found=false;

			$sql1= "select * from user where username=\"$_POST[username]\" or email=\"$_POST[email]\"";

			$query = $con->query($sql1);

			while ($r=$query->fetch_array()) {

				$found=true;

				break;

			}

			if($found){

				print "<script>alert(\"Nombre de usuario o email ya estan registrados.\");window.location='../registro.php';</script>";

			}

			$sql = "insert into user(username,fullname,email,password,created_at,secure) value (\"$_POST[username]\",\"$_POST[fullname]\",\"$_POST[email]\",\"$_POST[password]\",NOW())";

			$query = $con->query($sql);

			if($query!=null){

				print "<script>alert(\"Registro exitoso. Proceda a logearse\");window.location='../login.php';</script>";

			}

		}

	}

}

?>

$sql = "insert into user(username,fullname,email,password,created_at,secure) value (\"$_POST[username]\",\"$_POST[fullname]\",\"$_POST[email]\",\"$_POST[password]\",NOW(),\"$_POST[Security]\")";

$sql = "insert into user(username,fullname,email,password,created_at,secure) value ('".$_POST["username"]."', '".$_POST["fullname"]."', '".$_POST["email"]."', '".$_POST["password"]."', NOW(), '".$_POST["Security"]."')";

<?php

	$realname=$_POST['realname'];

	$mail=$_POST['nick'];

	$pass= $_POST['pass'];

	$rpass=$_POST['rpass'];

	$Security=$_POST['Security'];

	require("connect_db.php");

//la variable  $mysqli viene de connect_db que lo traigo con el require("connect_db.php");

	$checkemail=mysqli_query($mysqli,"SELECT * FROM login WHERE email='$mail'");

	$check_mail=mysqli_num_rows($checkemail);

	if($pass==$rpass){

		if($check_mail>0){

			echo ' <script language="javascript">alert("Atencion, ya existe el mail designado para un usuario, verifique sus datos");</script> ';

		}else{

			//require("connect_db.php");

//la variable  $mysqli viene de connect_db que lo traigo con el require("connect_db.php");

			mysqli_query($mysqli,"INSERT INTO login VALUES('','$realname','$pass','$mail','$Security','','2')");

			//echo 'Se ha registrado con exito';

			echo ' <script language="javascript">alert("Usuario registrado con éxito");</script> ';

		}

	}else{

		echo 'Las contraseñas son incorrectas';

	}

$query="INSERT INTO login VALUES('','$realname','$pass','$mail','$Security','','2')";

$r=mysqli_query($mysqli,$query) or die(mysqli_error($mysqli)." Q=".$query);

<?php

	$realname=$_POST['realname'];

	$mail=$_POST['nick'];

	$pass= $_POST['pass'];

	$rpass=$_POST['rpass'];

	$secure=$_POST['secure'];