data not passed from one page to another after login form validation

<?php
		
		//$msg = $_POST['msg'];
		$usernameerr = $passerr = "";
		$username = $pass = "";
		//echo "<p>" . $msg . "</p>";
		if ($_SERVER["REQUEST_METHOD"] == "POST")
		{	
			 
			if (empty($_POST["username"]))
			{
				$usernameerr = "Username is missing";
				$valid = false;
			}
			else
			{
			$username = test_input($_POST["username"]);
			$valid = true;
			}
   
			if (empty($_POST["password"]))
			{
				$passerr = "Password is missing";
				$valid = false;
			}
			else
			{
				$pass = test_input($_POST["password"]);
				$valid = true;
			}
			if($valid)
			{

				header('Location: http://localhost/tabs/validate_login.php');
				exit();
			}
		}

		function test_input($data)
		{
			$data = trim($data);
			$data = stripslashes($data);
			$data = htmlspecialchars($data);
			return $data;
		}
?>

<form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>" style= " margin-left:25%; " >
        <table  width= 100% height= 100%;>
            <tr>
                <td width = 60px><label for="username">Username</label></td>
                <td width = 60px><input type="text" 
                  name="username" id="username"></td>		
            </tr>
            <tr>
                <td width = 60px><label for="password">Password</label></td>
                <td width = 60px><input name="password" 
                  type="password" id="password"></input></td>
            </tr>
			<tr>
			</tr>
			<tr>
			</tr>
			<tr>
			</tr>
			<tr>
			</tr>
            <tr>
			</table>
			<table >
                <td width = 100px align="center"><input type="submit" value="Submit"/>
                <td width = 100px align = "center"><input type="reset" value="Reset"/>
            </tr>
        </table>
    </form>	

<?php

	// Grab User submitted information
	$username = $_POST["username"];
	$pass = $_POST["password"];

	
	
	// Connect to the database
	$con=mysqli_connect("localhost","root","Quest1234","maintenance_help");
	// Make sure we connected succesfully
	if(! $con)
	{
		die('Connection Failed'.mysql_error());
	}

	// Select the database to use
	$result = mysqli_query($con,"SELECT username, pass FROM login WHERE username = '$username'");
	$row = mysqli_fetch_array($result);
	$match  = mysql_num_rows($result);
	
	if($row["username"]==$username && $row["pass"]==$pass)
	{
	//echo $username;
	//echo $pass;
	//echo "<p>" . $row['username'] . "</p>";
	//echo "<p>" . $row['pass'] . "</p>";
		echo "success";
	//	echo "<p> apple " . $match . "</p>";
		//echo "<script> window.location.assign('stockform.php'); </script>";
	}
	else
	{
		//$msg = "Invalid Username or Password";
		//$_POST['msg'];
		//header('Location:  http://localhost/tabs/login.php');
		echo "invalid username or password";
	}
?>