If you use search form from default PW template, you could do the following with the above script mentioned.
<input type='text' name='q' id='search_query' value='<?php echo htmlentities($input->whitelist('q'), ENT_QUOTES, 'UTF-8'); ?>' />
Add HTML5 placeholder to input field like this:
<input type='text' name='q' id='search_query' placeholder='Search' value='<?php echo htmlentities($input->whitelist('q'), ENT_QUOTES, 'UTF-8'); ?>' />
Include the script above to your
head section and trigger it like this (also in the head):
<script>
$(document).ready(function() {
$('input[name="q"]').placeholder();
});
</script>