Minimalistic code to add rel=canonical links on HTML header

functions.php

<?php
/**
 *  Minimalistic code to add rel=canonical links on HTML header.
 *
 *  No wish to install a full-blown SEO plugin just to get those rel=canonical links? That's fine!
 *  Just place the code below somewhere on your child's theme `functions.php` file.
 *  If you also wish to get a meta description tag in the header, uncomment the last few lines.
 *
 *  @todo this only works for singulars (page, post, attachment page, etc.); not for archive/search pseudo-pages,
    which do *not* have a unique identifier at the $wp_query level... that's (mostly) why SEO plugins include
    a *lot* of code just to deal with those ...
 *
 *  @author Gwyneth Llewelyn
 *  @license MIT
 *  @revision 2
 *  
 **/

/**
 *  Attempt to get a canonical link on the header (gwyneth 20210526).
 **/
add_action('wp_head', 'my_canonical_link');
function my_canonical_link() {
	// needs to handle non-singular cases, which are a bit more tricky
	if ( is_front_page() ) {
		$canonical_url = get_home_url();
		if ( ! is_main_site() ) { // add trailing slash for subsite home directories
			$canonical_url = trailingslashit( $canonical_url );
		}
	} else {
		$canonical_url = wp_get_canonical_url();	// can return false if post is unpublished
	}
	if ( empty( $canonical_url ) ) {
		echo '<link rel="canonical" href="' . esc_url( $canonical_url ) . '" />' . PHP_EOL;
	}
	// Uncomment the code below if you *also* wish to add a meta description tag. WordPress does *not* place one
	//  by default; these days, they are pretty much useless for Google, but Bing and possibly Yandex still
	//  reads them (gwyneth 20210526)
	/*
	echo '<meta name="description" content="' . esc_html( is_single() ?
		single_post_title( get_bloginfo( 'name' ) . " - " , false ) . ( has_excerpt() ? " - " . get_the_excerpt() : '' )
		: get_bloginfo( 'name' ) . " - " . get_bloginfo( 'description' ) ) . '" />' . PHP_EOL;
	*/
}