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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";
	}
}

Uses

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

Determines whether the query is 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.

Changelog

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

© 2003–2019 WordPress Foundation
Licensed under the GNU GPLv2+ License.
https://developer.wordpress.org/reference/functions/rel_canonical