<?php
$connection = mysqli_connect('127.0.0.1','root','','test_db');
$result = mysqli_query($connection, "SELECT * FROM 'articles_categories'");
$r1 = mysqli_fetch_assoc($result);
?>
mysqli_query
выдал False
, т.е. ошибку.`
а не '
.$result = mysqli_query($connection, "SELECT * FROM `articles_categories`");