You could probably add a url parameter in your show all link like:
<a href="results.php?showall=1">show all</a>
Then on the top of the results page add code:
What are you using for the trigger for your filter on the results page? Assuming it is the button named "submitButtonName"
<?php
if (isset($_GET['showall'])) {
$_POST['submitButtonName'] = 1;
}
?>