Showing Co-Authors on post page

by

Your error is that you’ve written an if inside the sprintf argument list. You can’t do that. Instead, you could use the ternary operator, which has syntax

<condition> ? <result if true> : <result if false>

e.g.

$byline = sprintf(
        __('by %s'),
        function_exists('coauthors_posts_links')
            ? ('<span class="author vcard">' .
                    coauthors_posts_links( null, null, null, null, false ) . '</span>')
            : ('<span class="author vcard"><a class="url fn n" href="'.esc_url(
                    get_author_posts_url($author_id)
                ).'">'.esc_html(get_the_author_meta('display_name', $author_id)).
                '</a>'.'</span>')
    );

(brackets added for clarity – you probably don’t need them all)
or use a separate variable for the post links and set that first using an if-else, e.g.

if (function_exists('coauthors_posts_links')) {
    $author_post_links = coauthors_posts_links( null, null, null, null, false );
} else {
    $author_post_links="<a class="url fn n" href="" .
        esc_url( get_author_posts_url( $author_id ) ) .
        '">' . esc_html( get_the_author_meta( 'display_name', $author_id ) ) . '</a>';
}
$byline = sprintf(
    __('by %s'),
    '<span class="author vcard">' . $author_post_links . '</span>' );

As discussed in comments

  • by default coauthors_posts_links() will echo the links as well as returning them as a string, which is not what you want here. You’ll need to pass $echo = false, which is the fifth argument, so we’ll need to fill in the previous four with default null values.
  • I don’t think you want to esc_html() the output of coauthors_post_links() so I’ve removed that too.