Change canonical tag with custom field

• yvonne-sil

• March 17, 2022 at 12:39 pm

I would like to change the canonical tag for some posts using an ACF custom field.

I’ve created the field in ACF but having trouble figuring out how to insert it into the <head/> without using a plugin such as Yoast. I have found the following code to put in my functions.php but it is 7 years old. Can anyone confirm that this code would be safe to use?

/* Begin Rel-canonical by custom fields */
// A copy of rel_canonical but to allow an override on a custom tag
function rel_canonical_with_custom_tag_override()
{
 if( !is_singular() )
 return;

 global $wp_the_query;
 if( !$id = $wp_the_query->get_queried_object_id() )
 return;

 // check whether the current post has content in the "canonical_url" custom field
 $canonical_url = get_post_meta( $id, 'canonical_url', true );
 if( '' != $canonical_url )
 {
 // trailing slash functions copied from http://core.trac.wordpress.org/attachment/ticket/18660/canonical.6.patch
 $link = user_trailingslashit( trailingslashit( $canonical_url ) );
 }
 else
 {
 $link = get_permalink( $id );
 }
 echo "<link rel='canonical' href='" . esc_url( $link ) . "' />\n";
}

// remove the default WordPress canonical URL function
if( function_exists( 'rel_canonical' ) )
{
 remove_action( 'wp_head', 'rel_canonical' );
}
// replace the default WordPress canonical URL function with your own
add_action( 'wp_head', 'rel_canonical_with_custom_tag_override' );
/* End Rel-canonical by custom fields */

• John Huebner

• March 17, 2022 at 1:02 pm

I would not use that code.

The function rel_canonical() calls wp_get_canonical_url(). The value returned from this function is filterable. I would use this hook and create a filter to replace what WP is using. This is how the plugins you’ve mentioned to this.

• yvonne-sil

• March 17, 2022 at 1:16 pm

Thanks! There seems to be an ACF-related function on that hook page that looks like it might work so I will test that out.

function wpdocs_edit_canonical_urls( $original_url, $post ) {
    $override_url = get_post_meta( $post->ID, 'canonical_url', true );
 
    if ( empty( $override_url ) ) {
        return $original_url;
    }
 
    return $override_url;
 
}
 
add_filter( 'get_canonical_url', 'wpdocs_edit_canonical_urls', 10, 2 );