PHP - Wordpress - Search - wordpress custom search function that encompasses ACF/advanced custom fields and taxonomies and split expression before request. I updated this original script with better documentation and XSS / SQL injection support.

custom-search-acf-wordpress.php

/*
##############################
########### Search ###########
##############################
*/

/**
 *
 * [list_searcheable_acf list all the custom fields we want to include in our search query]
 * @return [array] [list of custom fields]
 * Included are steps to help make this script easier for other to follow
 * I also updated this work to include XSS and SQL injection projection
 * 1- Define list of ACF fields you want to search through - do NOT include taxonomies here
 * See step 8 for taxonomy inclusion
 */
function list_searcheable_acf(){
  $list_searcheable_acf = array(
		"your",
		"custom",
		"post-types",
		"here"
	);
  return $list_searcheable_acf;
}

/**
 * [advanced_custom_search search that encompasses ACF/advanced custom fields and taxonomies and split expression before request]
 * @param  [query-part/string]      $search    [the initial "where" part of the search query]
 * @param  [object]                 $wp_query []
 * @return [query-part/string]      $search    [the "where" part of the search query as we customized]
 * modified from gist: https://gist.github.com/FutureMedia/9581381/73afa809f38527d57f4213581eeae6a8e5a1340a
 * see https://vzurczak.wordpress.com/2013/06/15/extend-the-default-wordpress-search/
 * credits to Vincent Zurczak for the base query structure/spliting tags section
 */
 function advanced_custom_search( $search, &$wp_query ) {
     global $wpdb;

     if ( empty( $search )) {
  return $search;
     }

     // 1- get search expression
     $terms_raw = $wp_query->query_vars[ 's' ];

     // 2- check search term for XSS attacks
     $terms_xss_cleared = strip_tags($terms_raw);

     // 3- do another check for SQL injection, use WP esc_sql
     $terms = esc_sql($terms_xss_cleared);

     // 4- explode search expression to get search terms
     $exploded = explode( ' ', $terms );
     if( $exploded === FALSE || count( $exploded ) == 0 ) {
  $exploded = array( 0 => $terms );
     }

     // 5- setup search variable as a string
     $search = '';

     // 6- get searcheable_acf, a list of advanced custom fields you want to search content in
     $list_searcheable_acf = list_searcheable_acf();

     $table_prefix = $wpdb->prefix;

     // 7- search through tags, inject each into SQL query
     foreach( $exploded as $tag ) {
         $search .= "
           AND (
             (".$table_prefix."posts.post_title LIKE '%$tag%')
             OR (".$table_prefix."posts.post_content LIKE '%$tag%')
             OR EXISTS (
               SELECT * FROM ".$table_prefix."postmeta
                WHERE post_id = ".$table_prefix."posts.ID
                  AND (";
   // 7b - add each custom post-type into SQL query
         foreach ($list_searcheable_acf as $searcheable_acf) {
           if ($searcheable_acf == $list_searcheable_acf[0]) {
             $search .= " (meta_key LIKE '%" . $searcheable_acf . "%' AND meta_value LIKE '%$tag%') ";
           } else {
             $search .= " OR (meta_key LIKE '%" . $searcheable_acf . "%' AND meta_value LIKE '%$tag%') ";
           }
         }
  // 8- Add to search string info from comments and custom taxonomies
  // You would need to customize the taxonomies below to match your site
          $search .= ")
             )
             OR EXISTS (
               SELECT * FROM ".$table_prefix."comments
               WHERE comment_post_ID = ".$table_prefix."posts.ID
                 AND comment_content LIKE '%$tag%'
             )
             OR EXISTS (
               SELECT * FROM ".$table_prefix."terms
               INNER JOIN ".$table_prefix."term_taxonomy
                 ON ".$table_prefix."term_taxonomy.term_id = ".$table_prefix."terms.term_id
               INNER JOIN ".$table_prefix."term_relationships
                 ON ".$table_prefix."term_relationships.term_taxonomy_id = ".$table_prefix."term_taxonomy.term_taxonomy_id
               WHERE (
              taxonomy = 'your'
      OR taxonomy = 'custom'
      OR taxonomy = 'taxonomies'
      OR taxonomy = 'here'
            )
                AND object_id = ".$table_prefix."posts.ID
                AND ".$table_prefix."terms.name LIKE '%$tag%'
             )
         )";
     }
     return $search;
 }

// 9- use add_filter to put advanced_custom_search into the posts_search results
add_filter( 'posts_search', 'advanced_custom_search', 500, 2 );