PHP form error logic - php

Question

Normally when we submit form in php and show errors then the errors are posted in other page.
Now my question is.
If we want to show the errors on same page mean on the form page, then how to do it?
I used session for that and found it better then other process but when I want to show all session variable at once then it will not show, why?
My code is:
if(isset($_POST) && count($_POST)>0) {
$user =test_input($_POST['user']);
$pass = test_input($_POST['pass']);
$securepassword=hash('sha512', $pass);
if(empty($user)&&empty($pass))
{
echo 'fill every field';
}
else if(empty($user)||empty($pass))
{
echo 'fill both the field';
}
else
{
$sSQL = "SELECT * FROM signup WHERE Username ='".$user."' AND Password = '".$securepassword."'";
$result = mysql_query($sSQL) or die(mysql_error());
$row=mysql_num_rows($result);
if($row==1)
{
// Set username session variable
$_SESSION['user'] = $user;
// Jump to secured page
header('location:index.php');
}
else
{
header('location:signin.php');
}
}
}

Answer 1

You better use JavaScript for validating the form. Lets say this is your HTML code.
<form method="post" name="loginForm" action="abc.php">
<input type="text" name="user">
<input type="password" name="pass">
</form>
Now, let’s add validate this form before you POST to abc.php:
<form method="post" name="loginForm" action="abc.php" onsubmit="return validater()">
<input type="text" name="user">
<input type="password" name="pass">
</form>
And the JavaScript is:
function validater() {
var a= document.forms["loginForm"]["user"].value;
if (a==null || a=="") {
alert("Fill out the username");
return false;
}
var b= document.forms["loginForm"]["pass"].value;
if (b==null || b=="") {
alert("Enter your password");
return false;
}
}

Answer 2

Logic is simple you can put the php code and html code on the same page. In the following order.
Put your php validation code.
Put your error code to show errors above the form.
Put your html code to show the form fields on the page.
There is no need of session to show errors to the user.