<?php
if(isset($_POST['btnsubmit']))
{

if($_POST['username'] == "test" . "admin")
{
    echo "username correct!<br/>";
    $user = true;

}
else
{
    echo "username wrong!<br/>";
    $user = FALSE;
}
if($_POST['password'] == "test" . "admin")
    {
        echo "password correct!<br/>";
        $pass = true;
    }
    else
    {
        echo "password failed!<br/>";
        $pass = FALSE;
    }
    if($user == TRUE && $pass == TRUE)
    {
        echo "Login succeded!";
        echo "<meta HTTP-EQUIV='REFRESH' content='5; url= www.google.com'>";
    }
    else
    {
        echo "Login failed";
        echo '<script type="text/javascript">'
                , 'history.go(-1);'
            , '</script>'
            ;
    }
}
?>

$users = Array("admin" => "admin", "test" => "test");

if(isset($users[$_POST['username']]))
...
if($users[$_POST['username']] == $_POST['password'])

echo '<script type="text/javascript">'
                , 'history.go(-1);'
            , '</script>'
            ;

echo '<script type="text/javascript">'
            . 'history.go(-1);'
        . '</script>'
        ;

if($user == TRUE && $pass == TRUE)
    {
        echo "Login succeded!";
        echo "<meta HTTP-EQUIV='REFRESH' content='5; url= https://www.google.com'>";
    }