Keep Coding

Looking for some advice on this problem. I happened to run into using the php framework Yii, but seems more a universal issue with web forms/my lack of understanding this concept.
Issue: When pressing the back button on the browser, user gets "Document Expired" only if the form had a validation error. If there was no validation error before it was submitted, there is no "Document Expired" problem (because of the redirect).
Take this simple generic example of a controller:
public function actionSomething() {
$model=new ExampleClass;
if(isset($_POST['ExampleClass'])) {
$model->attributes=$_POST['ExampleClass'];
if($model->validate()) {
$this->redirect(Yii::app()->createUrl('some/where'));
}
}
$this->render('_form',array(
'model'=>$model
));
}
So when validation fails, the form is sitting there with $_POST data. Then, data is submitted again, validation passes and the redirect happens. Next, when the back button is pressed on the browser, it runs into the form in the state prior to passing validation and I get the "Document Expired" problem.
How can I prevent this?

I'm not totally sure how far or what you're willing to try - I might suggest using sessions to store variables. Doing this will allow you to recall the values of the variables during a page refresh.
http://us2.php.net/manual/en/session.examples.basic.php
Here's a better link for a usage demo:
http://www.w3schools.com/php/php_sessions.asp