Login validation

<html>
<head><title>Login Form</title></head>
<body>
Login form <br>
<form  method="get" action="">
Username
<input name="txtUsername" type="text"><br>
Password
<input name="txtPassword" type="password">
<br>
<input type="submit" value="Login">
</form>
<hr>
Error messages<br>
<?php
//simple if
if ($_GET['txtUsername']==null){
    echo "Pls provide username<br>";
}

//if-else example
if($_GET['txtPassword']==null){
    echo "Pls provide password";
}
else{
    //simple nested if
    //must be 8 characters
    if(strlen($_GET['txtPassword'])<8){
        echo "Password must be 8 chars";
    }
}
?>

</body>
</html>