Question

<?php
include 'lib/db_conn.php';
$uid=$_REQUEST['uid'];
$pass=$_REQUEST['pass'];
if(($uid==NULL && $pass==NULL) ||($uid==NULL) ||($pass==NULL))
{
    header("location:index.php?msg=Fields can't be left blank..");
}

$pass=md5($pass);
$sql1="SELECT * FROM `tb_user` WHERE `email`='$uid' AND `pass`='$pass'";
$rs1=mysql_query($sql1) or die (mysql_error());
$row1=mysql_fetch_array($rs1) or die (mysql_error());
$email=$row1['email'];
if($uid==$email)
{
        session_start();
        $_SESSION['id']=$row1['id'];
        header("location:home.php");
}
else
{
header("location:index.php?msg=Wrong Credentials..");
}
?>

Answer 1

it is better not to write a message in the address bar but just tokenize it, i.e.:

header("location:index.php?msg=wcred");

and in the index.php:

if ($_GET['msg'] == "wcred") echo "Wrong Credentials..";

And, as Kai mentioned, $uid=$_REQUEST['uid']; must be

$uid=mysql_real_escape_string($_REQUEST['uid']);

Also, as dnagirl mentioned, field emptiness checking is wrong.
Also, as I am to mention, exit must follow any location header

<?php
if((empty($_REQUEST['uid']) OR empty($_REQUEST['pass'])) {
    header("location:index.php?msg=fempty");
    exit;
}
include 'lib/db_conn.php';

$pass=md5($_REQUEST['pass']);
$pass=mysql_real_escape_string($pass);
$uid=mysql_real_escape_string($_REQUEST['uid']);

$sql1="SELECT * FROM `tb_user` WHERE `email`='$uid' AND `pass`='$pass'";
$rs1=mysql_query($sql1) or die (mysql_error());
$row1=mysql_fetch_array($rs1) or trigger_error(mysql_error());
if($row1) {
    session_start();
    $_SESSION['id']=$row1['id'];
    header("location:home.php");
    exit;
} else {
    header("location:index.php?msg=wcred");
    exit;
}
?>