rel_canonical()

Outputs rel=canonical for singular queries.

Source

File: wp-includes/link-template.php 

function rel_canonical() {
	if ( ! is_singular() ) {
		return;
	}

	$id = get_queried_object_id();

	if ( 0 === $id ) {
		return;
	}

	$url = wp_get_canonical_url( $id );

	if ( ! empty( $url ) ) {
		echo '<link rel="canonical" href="' . esc_url( $url ) . '" />' . "\n";
	}
}

Expand Source Code View on GitHub

Related

Uses

Uses
Uses Description
wp-includes/query.php: is_singular()

Is the query for an existing single post of any post type (post, attachment, page, custom post types)?
wp-includes/query.php: get_queried_object_id()

Retrieve ID of the current queried object.
wp-includes/link-template.php: wp_get_canonical_url()

Returns the canonical URL for a post.
wp-includes/formatting.php: esc_url()

Checks and cleans a URL.

Changelog

Changelog
Version Description
WP-4.6.0 Adjusted to use wp_get_canonical_url().
WP-2.9.0 Introduced.