if (is_single()) { ?>
<?php
$next_post = get_next_post();
if($next_post) {
  $next=get_permalink($next_post->ID);
}
$prev_post = get_previous_post();
if($prev_post) {
  $prev=get_permalink($prev_post->ID);
}
  ?>
<script type="text/javascript">
  document.onkeydown = checkKey;
function checkKey(e) {
    e = e || window.event;
    if (e.keyCode == '37') {
        // left arrow
        <?php echo 'window.location.assign("'.$prev.'");'; ?>
    }
    else if (e.keyCode == '39') {
        <?php echo 'window.location.assign("'.$next.'");'; ?>
    }
}
</script>
<?php  {