logic question with a form

I want my PHP code to display and handle a form.

If the form hasn't been submitted it needs to display the form.

If the form has been submitted but the data couldn't be validated then I want to display the same form (with the user's data intact).

If the form has been submitted and the data was validated then it handles the form and prints a message.

The problem I seem to be having is that I have nested conditionals that both need to execute the same code.  I cant seem to wrap my head around it.

Form/form processing pseudo code -

// form processing code
if(form_submitted){

validate form data

if(no form_errors){
	// use form data
}
}

// form code
if(form_not_submitted OR form_errors){
if(form_errors){
	display form errors
}
display form with existing field values (if present)
}

Example Form/form processing code -

<?php
// form processing code
if($_SERVER['REQUEST_METHOD'] == "POST"){

//validate form data
$form_errors = array(); // define array to hold form errors

if($_POST['username'] == ''){
	$form_errors[] = 'No username supplied!';
}
if($_POST['password'] == ''){
	$form_errors[] = 'No password supplied!';
} else {
	// further password validation
	if(strlen($_POST['password']) < 5){
		$form_errors[] = "Password length must be at least 5 characters!";
	}
}

if(empty($form_errors)){
	// use form data
	echo "Thank you for submitting your data, " . htmlentities($_POST['username'],ENT_QUOTES) . ".<br />";
}
}

// form code
if($_SERVER['REQUEST_METHOD'] != "POST" || !empty($form_errors)){
if(!empty($form_errors)){
	//display form errors
	echo "Please correct the following form errors -<br />";
	foreach($form_errors as $error){
		echo "$error<br />";
	}
}
//display form with existing field values (if present)
?>
<form method='post' action=''>
<label for="username">Username:</label>
<input type='text' id='username' name='username' value='<?php echo isset($_POST['username']) ? htmlentities($_POST['username'],ENT_QUOTES) : ''?>'>
<br />
<label for="password">Password:</label>
<input type='password' id='password' name='password' value='<?php echo isset($_POST['password']) ? htmlentities($_POST['password'],ENT_QUOTES) : ''?>'>
<br />
<input type='submit'>
</form>
<?php
}
?>