I understand how to use get_post_metadata(), however this code in a custom RSS feed template:

while( have_posts()) : the_post();

    // Has a custom URL been supplied? Use that in preference.
    $feature_permalink = ‌‌get_post_meta(get_the_ID(), '_featureurl', true);


is giving:

HP Fatal error: Uncaught Error: Call to undefined function ‌‌get_post_meta() in /srv/www/foo/htdocs/wp-content/themes/bar/feed-feature.php

(and indeed the PhpStorm editor highlights it accordingly, but with an xdebug breakpoint set, I can run it fine in the debug console. All the other various feed function calls - for the standard permalink, title, guid etc. - work fine.)

The template is in functions.php, called with:

function foo_custom_rss() {
    if ( in_array('feature', get_query_var('post_type')) ) {
        get_template_part( 'feed', 'feature' );
    } else {
        get_template_part( 'feed', 'rss2' );

remove_all_actions( 'do_feed_rss2' );
add_action( 'do_feed_rss2', 'foo_custom_rss', 10, 1 );
  • What specifically are you trying to add/modify on the feature post type's RSS2 feed? – Dave Romsey Mar 4 '18 at 6:40

To modify an RSS item's <link> node value, use the the_permalink_rss filter. This is demonstrated in the code below where if a feature post has a value set for _featureurl, we will return the custom value. Otherwise, the default permalink is returned:

 * Filters the permalink to the post for use in feeds.
 * @param string $post_permalink The current post permalink.
add_filter( 'the_permalink_rss', 'wpse_the_permalink_rss', 10, 1 );
function wpse_the_permalink_rss( $post_permalink ) {
    // Bail if this is not a feature.
    if ( 'feature' !== get_query_var( 'post_type') ) {
        return $post_permalink;

    // Get the permalink URL.
    $feature_permalink = get_post_meta( 

    // If the the custom URL has been specified return it, otherwise, use default permalink.
    // Note: This is run through esc_url() via the_permalink_rss().
    if ( $feature_permalink  ) {
        return $feature_permalink; 
    } else {
        return $post_permalink;     

Example RSS output for the feature post type, which has the following URL:


Note that the <link> node under the <item> node contains the URL https://google.com, which is what I set for the _featureurl custom field:

<?xml version="1.0" encoding="UTF-8"?><rss version="2.0"

    <title>Features &#8211; WP Theme Testing</title>
    <atom:link href="http://localhost/wp-theme-testing/feed/?post_type=feature" rel="self" type="application/rss+xml" />
    <description>I &#60;3 testing themes!</description>
    <lastBuildDate>Sun, 04 Mar 2018 05:46:16 +0000</lastBuildDate>

    <title>Features &#8211; WP Theme Testing</title>
        <title>test feature</title>

        <pubDate>Sun, 04 Mar 2018 05:41:29 +0000</pubDate>

        <guid isPermaLink="false">http://localhost/wp-theme-testing/?post_type=feature&#038;p=3035</guid>
        <description><![CDATA[a test!]]></description>
                <content:encoded><![CDATA[<p>a test!</p>
|improve this answer|||||
  • Hello - thanks. I should clarify, I’m not adding a new node, only changing the value of the permalink if the post has that custom post meta value set. I did actually take a direct copy of feed-rss2.php and drop it in my theme folder (renamed), but I can’t call get_post_meta() in it for some reason, if you’ve any idea why that might be.. – William Turrell Mar 4 '18 at 8:10
  • Hi and thank you for the feedback. I've updated my answer. The values of the default nodes can be manipulated using various filters. get_post_meta() can be used within callbacks attached to these filters or within the rss2_item() action. If get_post_meta() is called within the RSS template directly, it will trigger an error because that portion of WP has not been loaded at that point. – Dave Romsey Mar 4 '18 at 8:50
  • 1
    This all makes sense (and I've up-voted) however I still get undefined for get_post_meta() when it's used in functions.php... – William Turrell Mar 4 '18 at 12:33
  • Hrmm, I think something is wrong on your end. I tested this code before posting it and it works. I added the output example to my answer. Try clearing your browser's cache and removing the foo_custom_rss() related code from your theme. – Dave Romsey Mar 4 '18 at 15:42

This is a workaround, but it did solve things for me.

First I moved this site from my vvv to vvv2 dev environment, to rule out anything odd there. Then I went through the WP config files and the rest of functions.php carefully looking for anything with the potential to break things.

No luck so far.

I traced get_post_meta() in the source, thinking I might only be able to solve this by rewriting an equivalent function myself (creating a DB query if necessary) - as readers may know it's simply a wrapper for get_metadata() - for some reason this does work.

So if anyone can suggest why get_post_meta() is undefined and get_metadata() isn't...

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