Echo PHP message into a specific preloaded div?

Question

I'm having trouble getting an error message to output on the page I have already loaded. After the Log In button is pressed, I would like to check the input information, and if it is not correct return an error in a specific location without reloading the page. Is it possible to echo inside an element (such as inside my div as I am trying to do below)?

<?php
if (isset($_POST['login'])) { login(); }
function login(){
    if(isset($_POST['username'])&&isset($_POST['password'])) {
        $username = $_POST['username'];
        $password = $_POST['password'];
        $password_hash=md5($password);

        if(!empty($username)&&!empty($password)) {
            //stuff
        } else {
            $loginerror = "You must supply a username and password";
        }
    }
}
?>

<div style="z-index: 10; float:right; margin-top: -20;">
    <h3>Already a user? Log in:</h3>
    <div><?php if (isset($loginerror)){echo $loginerror;} ?></div>
    <form action="<?php echo $current_file; ?>" method="POST">
        <div style="float: left; width: 80px;">Username:</div> <input type="text" name="username"> <br />
        <div style="float: left; width: 80px;">Password:</div> <input type="password" name="password"> 
        <input type="submit" value="Log In" name="login">
    </form>
</div>

Answer 1

Because you are not supplying the form data to the login function, it can't verify anything. I was able to modify your code and make it smaller, but it doesn't utilize a login function.

<?php
$loginerror = "";

if(isset($_POST['login'])) {

    if( !empty($_POST['username']) && !empty($_POST['password']) ) {
        //stuff
    }
    else {
        $loginerror = "You must supply a username and password.";
    }

}

?>

<div style="z-index: 10; float:right; margin-top: -20;">
    <h3>Already a user? Log in:</h3>
    <div><?php echo $loginerror; ?></div>
    <form action="" method="POST">
        <div style="float: left; width: 80px;">Username:</div> <input type="text" name="username"> <br />
        <div style="float: left; width: 80px;">Password:</div> <input type="password" name="password"> 
        <input type="submit" value="Log In" name="login">
    </form>
</div>

Now whenever you need to give the user a login error, simply use $loginerror = "error here";