What is the difference between posts_per_page and numberposts in WordPress

This post was written both for myself and for anyone who's ever wondered — what's the difference between the “numberposts” and “posts_per_page” parameters passed to the get_posts function?

I sometimes find myself trying to recall the difference, and in various places I use one or the other.

But what’s the actual difference? For that, let’s take a look at the source code of the WordPress “get_posts” function (luckily, it’s not very large):

function get_posts( $args = null ) {
	$defaults = array(
		'numberposts' => 5,
		'category' => 0, 'orderby' => 'date',
		'order' => 'DESC', 'include' => array(),
		'exclude' => array(), 'meta_key' => '',
		'meta_value' =>'', 'post_type' => 'post',
		'suppress_filters' => true
	);
	$r = wp_parse_args( $args, $defaults );
	if ( empty( $r['post_status'] ) )
		$r['post_status'] = ( 'attachment' == $r['post_type'] ) ? 'inherit' : 'publish';
	if ( ! empty($r['numberposts']) && empty($r['posts_per_page']) )
		$r['posts_per_page'] = $r['numberposts'];
	if ( ! empty($r['category']) )
		$r['cat'] = $r['category'];
	if ( ! empty($r['include']) ) {
		$incposts = wp_parse_id_list( $r['include'] );
		$r['posts_per_page'] = count($incposts);  // only the number of posts included
		$r['post__in'] = $incposts;
	} elseif ( ! empty($r['exclude']) )
		$r['post__not_in'] = wp_parse_id_list( $r['exclude'] );

	$r['ignore_sticky_posts'] = true;
	$r['no_found_rows'] = true;

	$get_posts = new WP_Query;
	return $get_posts->query($r);
}

As you can see, the “numberposts” key is simply a wrapper for “posts_per_page”:

if ( ! empty($r['numberposts']) && empty($r['posts_per_page']) )
	$r['posts_per_page'] = $r['numberposts'];

Now let’s check the “WP_Query” object and look for any mention of the “numberposts” parameter.
Checked it? Took a look? Right — there are no mentions of it. But “posts_per_page” is used.

From everything stated above, we can conclude that “numberposts” was introduced just for convenience.
Whether you use “numberposts” or “posts_per_page” — is simply a matter of personal preference.