Preview page just involves displaying the post variables.
<form method="post" action="previewPage">
<input type="text" name="element1">
<input type="text" name="element2">
<input type="submit">
</form>
When the above form is submitted,
you get the submitted values from the post array:
$element1 = $POST['element1'];
$element2 = $POST['element2'];
Now you can use the values anyway you want. You can display them, you can pre-populate the form like this:
<form method="post" action="previewPage">
<input type="text" name="element1" value="<?=$element1?>">
<input type="text" name="element2" value="<?=$element1?>">
<input type="submit">
</form>
