PHP - Login funciona con Wamp y no en servidor

<?php require_once('funciones/funciones.php'); ?>

<?php header('Content-Type: text/html; charset=UTF-8'); ?>

<?php

$conexion = mysql_pconnect('145.14.12.94', 'lperez', 'r3t0r1c4') or trigger_error(mysql_error(),E_USER_ERROR);

mysql_select_db('Registros', $conexion);

session_start();

$username = $_POST['Usu'];

$password = $_POST['Pass'];

$nivel = $_POST['Nivel'];

$tabla = mysql_query("SELECT * FROM Usuarios");

$totalRows_Datos = mysql_num_rows($tabla);

while ($registro = mysql_fetch_array($tabla))

{

	if (Desencriptar($registro['Usuario']) == $username and Desencriptar($registro['Contra']) == $password and Desencriptar($registro['Nivel']) == $nivel)

		{

		 $_SESSION['ID'] = $registro['ID'];

		 mysql_close($conexion);

		 header('Location: perfil.php');

		 }

		else

		{

			mysql_close($conexion);

		 header ('Location: acceso_error.php');

		 }

}

?>

<meta http-equiv="content-type" content="text/html; charset=utf-8"/>

<?php require_once('funciones/funciones.php'); ?>

<?php require_once("Connections/conexion.php");?>

<?php header('Content-Type: text/html; charset=UTF-8'); ?>

<?php if (!isset($_SESSION)) {

  session_start();

}?>

<?php

$username = $_POST['Usu'];

$password = $_POST['Pass'];

$nivel = $_POST['Nivel'];

$consulta="SELECT * FROM Usuarios";

if ($resultado=$conexion->query($consulta))

{

		while ($registro = $resultado -> fetch_array())

		{

			if (Desencriptar($registro['Usuario']) == $username and Desencriptar($registro['Contra']) == $password and Desencriptar($registro['Nivel']) == $nivel)

				{

				 $_SESSION['ID'] = $registro['ID'];

				 $conexion->close();

				 header('Location: perfil.php');

				 }

				else

				{

				  $conexion->close();

				 header ('Location: acceso_error.php');

				 }

		}

}

?>

<?php if (!isset($_SESSION)) {

  session_start();

}?>

<?php

$hostname_conexion = "1145.14.12.94";

$database_conexion = "Registros";

$username_conexion = "lperez";

$password_conexion = "r3t0r1c4";

$conexion = new mysqli($hostname_conexion, $username_conexion, $password_conexion,$database_conexion);

?>

$servidor="1145.14.12.94";

$db="Registros";

$user="lperez";

$clave="r3t0r1c4";

$conexion = new PDO("mysql:host=$servidor;dbname=$db", $user, $clave);