IMPORTANT: Make sure you are using the right plugin. Next/Previous Page Link Plus is intended for use in page templates. Next/Previous Post Link Plus is intended for use in single post templates. The two plugins have similar sounding, but different, function names. If you mistakenly install the wrong plugin, you will get a “call to undefined function” error. If you want to create next/previous links for your posts (including custom post types), please check out: https://wordpress.org/extend/plugins/ambrosite-nextprevious-post-link-plus/ This plugin creates two new template tags — next_page_link_plus and previous_page_link_plus — which may be used to generate next/previous navigation links for pages. The new tags include the following options: Sort the next/previous page links on any column, including alphabetically, by date, and by menu_order. Sort the next/previous links on custom fields (both string and integer sorts are supported). Loop around to the first page if there is no next page (and vice versa). Retrieve the first/last page, rather than the previous/next page (for First|Previous|Next|Last navigation links). Display the featured image alongside the links (WordPress 2.9 or higher). Truncate the link titles to any length, and display custom text in the tooltip. Display the title, date, author, and meta value of the next/previous links. Specify a custom date format for the %date variable. Restrict next/previous links to same parent page, author, or custom field...