Thanks. It works. I realise now this is just a php program generating a js script as its html output.
(Note to self - <?= $jboxid ?> is shorthand for <?php echo $jboxid; ?> - and this just echos the value of the variable into the script.)
I have a universal snippet for some js code within a <script> tag. It is essentially:
<script>new juicebox({configUrl : 'Jbox-D01992.xml'})</script>
If I put [wbcr_snippet id="4883"] in my webpage it works fine.
I want to make the config an attribute so I have changed the snippet to...