WordPress at a glance
function is not described

adjacent_post_link() WP 2.5.0

Displays the adjacent post link.

Can be either next post link or previous.

No Hooks.

Return

Nothing (null).

Usage

adjacent_post_link( $format, $link, $in_same_term, $excluded_terms, $previous, $taxonomy );
$format(string) (required)
Link anchor format.
$link(string) (required)
Link permalink format.
$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 category 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 adjacent post link: wp-includes/link-template.php WP 5.2.2

<?php
function adjacent_post_link( $format, $link, $in_same_term = false, $excluded_terms = '', $previous = true, $taxonomy = 'category' ) {
	echo get_adjacent_post_link( $format, $link, $in_same_term, $excluded_terms, $previous, $taxonomy );
}