PHP - Php Inside Variable To Echo Html?

I'm redoing my login script using functions and a basic switch function checking against a $_GET variable (hope you guys know what I'm talking about).

What I want to do is create two functions:

1 that displays the login form
and 1 that processes the information

The form tag would look like this:
<form action=\"<?php $_SERVER['PHP_SELF']?>?action=process\" method=\"post\">
</form>

Here's my switch statement:
Code: [Select]
<?php

//****************************************
//****************Action******************
//****************************************

switch ($_GET["action"]) {
default:
case "index":
if (!$_SESSION["member"]) {
if (!$timeout) {
display_form();
} else {
echo $timeout_error;
}
} else {
echo "You are already logged in.";
}
break;
case "process":
if (!$_SESSION["member"]) {
if (!$timeout) {
process_form();
} else {
echo $timeout_error;
}
} else {
echo "You are already logged in.";
}
break;
}

?>

This runs as soon as "login.php" loads.

It'll automatically run the commands under case "default" and "index". It'll first check to see if the member's logged in. If not, it'll then check to see if "$timeout" is true ($timeout becomes true if the member has attempted to login 5 times and failed). If not, it'll display the login form, by running "display_form()".

Once the form has been filled and submitted, the commands under case "process" will be performed. Again it will first check to see if the member's logged in. If not, it'll check to see if "$timeout" is true. If not, it'll start validating the forms.

For the validation, I've created a variable called "$errors_found", and scripted one if statement checking to see if the email and password exist. If so, the variables "$rm_field_un" and "$rm_field_pw" become true, as well as "$errors_found".

So, back to submission of the form...

If "$errors_found" is true, display the form (when the form is displayed, there will be an if statement within that says "<?php if ($rm_field_un) { echo "Username is wrong."; } ?>", which will be displayed right underneath the username field and label. Same with the password elements). If "$errors_found" is not true, go ahead and register the member.

Now, here's where I need help, because I'm really confused as to how to accomplish this.

The "display_form()" function will contain a single variable called "$login_form". It will contain a value of the HTML constructed login form, and the function will return the variable.

Remember how I stated in the third paragraph up that I will have <php> statements within the form which would display errors if necessary? Well, how do I put those if statements within the HTML, which is contained within the variable?

If that last question confuses you, allow me to present you with an instance:
Code: [Select]
<?php

function display_form() {
$login_form = "
<form name=\"login_form\" method=\"post\" action=\"<?php $_SERVER['PHP_SELF']?>?action=process\">
Username:
<input type=\"text\">
<?php if ($rm_field_un) { echo "Username is wrong"; } ?>
";

return $login_form;
}

?>

See what I mean?

This code confuses me ALOT. I'm not sure if the <php> tags are needed as it is already contained within existing ones, or what. Can somebody please help me out?

Any and all help is much appreciated. =D