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function is not described

get_adjacent_post_rel_link() WP 2.8.0

Retrieves the adjacent post relational link.

Can either be next or previous post relational link.

Hooks in function
Return

String/null. The adjacent post relational link URL.

Usage

get_adjacent_post_rel_link( $title, $in_same_term, $excluded_terms, $previous, $taxonomy );
$title(string)
Link title format.
Default: '%title'
$in_same_term(true/false)
Whether link should be in a same taxonomy term.
Default: false
$excluded_terms(array/string)
Array or comma-separated list of excluded term IDs.
Default: ''
$previous(true/false)
Whether to display link to previous or next post.
Default: true
$taxonomy(string)
Taxonomy, if $in_same_term is true.
Default: 'category'

Code of get adjacent post rel link: wp-includes/link-template.php WP 5.2.1

<?php
function get_adjacent_post_rel_link( $title = '%title', $in_same_term = false, $excluded_terms = '', $previous = true, $taxonomy = 'category' ) {
	if ( $previous && is_attachment() && $post = get_post() ) {
		$post = get_post( $post->post_parent );
	} else {
		$post = get_adjacent_post( $in_same_term, $excluded_terms, $previous, $taxonomy );
	}

	if ( empty( $post ) ) {
		return;
	}

	$post_title = the_title_attribute(
		array(
			'echo' => false,
			'post' => $post,
		)
	);

	if ( empty( $post_title ) ) {
		$post_title = $previous ? __( 'Previous Post' ) : __( 'Next Post' );
	}

	$date = mysql2date( get_option( 'date_format' ), $post->post_date );

	$title = str_replace( '%title', $post_title, $title );
	$title = str_replace( '%date', $date, $title );

	$link  = $previous ? "<link rel='prev' title='" : "<link rel='next' title='";
	$link .= esc_attr( $title );
	$link .= "' href='" . get_permalink( $post ) . "' />\n";

	$adjacent = $previous ? 'previous' : 'next';

	/**
	 * Filters the adjacent post relational link.
	 *
	 * The dynamic portion of the hook name, `$adjacent`, refers to the type
	 * of adjacency, 'next' or 'previous'.
	 *
	 * @since 2.8.0
	 *
	 * @param string $link The relational link.
	 */
	return apply_filters( "{$adjacent}_post_rel_link", $link );
}