<?php
$categories_q = mysqli_query($connection, "SELECT * FROM
`articles_categories` ");
$categories = array();
while( $cat = mysqli_fetch_assoc($categories_q) )
{
    $categories[] = $cat;
}
?>


<?php
  foreach ( $categories as $cat )
{
?>
<li><a href="/articles.php?categorie=<?php echo $cat['id']; ?>"><?php echo
$cat['title']; ?></a></li>
<?php
}
?>

$query = 'SELECT * FROM articles WHERE category_id =' . $_GET['categorie'];
$articles_from_cat = mysqli_query($connection , $query);

if ($_GET['categorie']) { // если запрос содержит переменную categorie (category/categories)
    $articles = mysqli_query($connection, "SELECT * FROM `articles` WHERE `category_id` = '" . (int)$_GET['categorie'] . "'"); // тащите посты с переданным id категории.
}