Pasting this PHP breaks my page. Why?

Question

I tried a few echo commands, and they return properly.

Breaking Statement

<?php 
$gallery_ids = $wpdb->get_results(
"SELECT gid FROM ".$wpdb->prefix."ngg_gallery ORDER BY gid ASC", ARRAY_A);
?>

This causes anything after this statement not to show up, consequentially breaking the page.

If I do things like this :

<?php
$custom_query = new WP_Query( 'posts_per_page=3&cat=1' );
if ( $custom_query->have_posts() ) : while ( $custom_query->have_posts() ) : $custom_query->the_post(); ?>

<p id="title"><?php the_title(); ?></p>
<p id="small-date">Posted on: <?php the_time('F j, Y') ?></p>
<?php the_excerpt(); ?>
<hr />

It works fine. So I know it can take complicated PHP statements.

I'm curious why my previous statement would break..

Answer 1

Make sure you reference the global $wpdb variable first, otherwise PHP doesn't know what $wpdb is or whether or not it has a get_results() method or a prefix property.

So just add global $wpdb; to fix it:

<?php 
global $wpdb;
$gallery_ids = $wpdb->get_results(
"SELECT gid FROM ".$wpdb->prefix."ngg_gallery ORDER BY gid ASC", ARRAY_A);
?>