Automatically link post titles

automatically-link-post-titles.php

<?php
if ( ! defined( 'ABSPATH' ) ) exit; // Exit if accessed directly

/**
 * Get post titles.
 *
 * Get a list of the post titles and related post IDs.
 *
 * @since 1.0.0
 *
 * @return	array	List of post titles/ post IDs.
 */
function wpal_get_post_titles() {

	$post_titles = array();

	if ( false === $post_titles = get_transient( 'wpal_post_titles' ) ) :

		global $wpdb;
		$post_titles = $wpdb->get_results( ( "SELECT ID, post_title from $wpdb->posts WHERE post_type='post' AND post_status = 'publish' AND CHAR_LENGTH(post_title) > 5" ) );

		set_transient( 'wpal_post_titles', $post_titles, WEEK_IN_SECONDS );

	endif;

	return $post_titles;

}


/**
 * Replace content.
 *
 * Search the content for the post titles and replace them when found.
 *
 * @since 1.0.0
 *
 * @param	string	$content	Existing post content.
 * @return	string				Modified post content.
 */
function wpal_replace_the_content( $content ) {

	// Prepare pot titles
	$post_titles = wpal_get_post_titles();
	$titles = wp_list_pluck( $post_titles, 'post_title', 'ID' );
	$preg_quote_titles = array();

	// Escape post titles
	foreach ( $titles as $key => $title ) :
		$preg_quote_titles[ $key ] = preg_quote( $title, '/' );
	endforeach;

	// Search for any post titles
	$content = preg_replace_callback( '/(' . implode( '|', $preg_quote_titles ) . ')/i', function( $matches ) use ( $preg_quote_titles ) {

		global $post;
		
		// Get post ID 
		$post_id = array_search( strtolower( $matches[0] ), array_map( 'strtolower', $preg_quote_titles ) );
		
		// Don't link to the current page
		if ( isset( $preg_quote_titles[ $post_id ] ) && $post->ID !== $post_id ) :
			
			// Use the post title from the DB to its casing is the same
			return '<a href="' . get_permalink( $post_id ) . '" class="post-link">' . $preg_quote_titles[ $post_id ] . '</a>';
			
		endif;

		return $matches[0];

	}, $content );

	return $content;

}
add_filter( 'the_content', 'wpal_replace_the_content' );


/**
 * Remove transient on save.
 *
 * Remove the transient holding the post_titles every time a
 * post is saved to ensure no unwanted links occur.
 *
 * ALSO re-index the post titles.
 *
 * @since 1.0.0
 */
function wpal_remove_transient() {

	// Delete old post title transient
	delete_transient( 'wpal_post_titles' );

	// Re-index post titles
	wpal_get_post_titles();

}
add_action( 'save_post', 'wpal_remove_transient' );

How do I actually implement this on a wordpress site? Do I have to copy it to functions.php? Do I have to somehow tell the site to load this file?

Hi, thanks for the function. For me it works with some changes :

Line 59 :

$post_id = array_search( strtolower( $matches[0] ), array_map( 'strtolower', $preg_quote_titles ) );

Become :

$post_id = array_search( strtolower( preg_quote ($matches[0] ) ), array_map( 'strtolower', $preg_quote_titles ) );

Line 65 :

return '<a href="' . get_permalink( $post_id ) . '" class="post-link">' . $preg_quote_titles[ $post_id ] . '</a>';

become :

return '<a href="' . get_permalink( $post_id ) . '" class="post-link">' . addslashes($preg_quote_titles[ $post_id ]) . '</a>';