[WordPress] An example for how to change the WHERE clause on the search page by using the WordPress API.

00-change-the-where-clause.php

<?php

add_filter( 'posts_where', 'acme_search_post_categories' );
/**
 * Includes the post meta table in the query that's using the search string.
 *
 * @param     string $where    The initial WHERE clause.
 * @return    string $where    The clause for including the post meta table, if on the search template.
 */
function acme_search_post_categories( $where ) {

	if ( is_admin() || ! is_search() ) {
		return $where;
	}

	$where =
		preg_replace(
			"/\(\s*wp_posts.post_title\s+LIKE\s*(\'[^\']+\')\s*\)/"	,
			'(wp_posts.post_title LIKE $1) OR
			 (wp_posts.post_content LIKE $1) OR
			 (wp_postmeta.meta_value LIKE $1)',
			$where
		);

	return $where;

}