PHP var into Javascript [duplicate]

This question already has an answer here:

• How to pass variables and data from PHP to JavaScript? 17 answers

I have this php var, and I want to send it to php, but this way I can't

<?php
    $nb = rand(1,68);
?>
///////////////////////////////////////////////////////////////////////////
<script type="text/javascript"> jQuery ( '#dwDB-main' ).fadeTo ( 1, 0.01 );
    function random() {
    $rd = <?php $nb ?>;
    return $rd;
    }