single.php: how to list all articles of current category in sidebar

This tag may be used outside The Loop by passing a post id as the parameter.

https://codex.www.ads-software.com/Function_Reference/get_the_category

try:
<?php foreach((get_the_category($post->ID)) as $category) { ?>

Thank you, alchymyth. I replaced <?php foreach((get_the_category()) as $category) { ?> with that code. The code still works, but the behaviour of the menu did not change in any way.

i missed this one as well:
if you are using single quotes with the parameters, you need to use a different way to include variables;

instead:

$myposts = get_posts('category=.$catVal&showposts=999&order=ASC');

try:

$myposts = get_posts('category='.$catVal.'&numberposts=999&order=ASC');

Thank you! That did the trick. I did suspect that that line is at fault, but had no idea what to do with it. ??

The code above is slightly wrong. Here’s a cleaner version of it:

<ul>
<?php foreach((get_the_category($post->ID)) as $category) { ?>

<?php $catVal = $category->cat_ID; }
   $IDOutsideLoop = $post->ID;
   global $post;
$myposts = get_posts('category='.$catVal.'&numberposts=999&order=ASC');
   foreach ($myposts as $post) { ?>
     <li<?php if($IDOutsideLoop == $post->ID) { echo " class=\"current\""; } ?>>
     <a href="<?php the_title(); ?>"><?php the_title(); ?></a>
<?php }; ?>
</ul>

Does anyone have any idea how I can make this link list look the same as the normal sidebar menu?! The one above is unformatted, so it looks different to the menu below it! Any one?! Cheers