How to search a specific custom post type and the ACF fields within

acf_custom_search.php

<?php
/**
 * [list_searcheable_acf list all the custom fields we want to include in our search query]
 * This could easily be something that is added to the admin as a plugin so that users can 
 * add searchable fields in an options table
 * @return [array] [list of custom fields]
 */
function list_searcheable_acf(){
  $list_searcheable_acf = array("title", "sub_title", "excerpt_short", "excerpt_long", "xyz", "myACF");
  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]      $where    [the initial "where" part of the search query]
 * @param  [object]                 $wp_query []
 * @return [query-part/string]      $where    [the "where" part of the search query as we customized]
 * 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( $where, &$wp_query ) {
    global $wpdb;
 
    if ( empty( $where ))
        return $where;
 
    // get search expression
    $terms = $wp_query->query_vars[ 's' ];
    
    // explode search expression to get search terms
    $exploded = explode( ' ', $terms );
    if( $exploded === FALSE || count( $exploded ) == 0 )
        $exploded = array( 0 => $terms );
         
    // reset search in order to rebuilt it as we whish
    $where = '';
    
    // get searcheable_acf, a list of advanced custom fields you want to search content in
    $list_searcheable_acf = list_searcheable_acf();
    foreach( $exploded as $tag ) :
        $where .= " 
          AND (
            (wp_posts.post_title LIKE '%$tag%')
            OR (wp_posts.post_content LIKE '%$tag%')
            OR EXISTS (
              SELECT * FROM wp_postmeta
	              WHERE post_id = wp_posts.ID
	                AND (";
        foreach ($list_searcheable_acf as $searcheable_acf) :
          if ($searcheable_acf == $list_searcheable_acf[0]):
            $where .= " (meta_key LIKE '%" . $searcheable_acf . "%' AND meta_value LIKE '%$tag%') ";
          else :
            $where .= " OR (meta_key LIKE '%" . $searcheable_acf . "%' AND meta_value LIKE '%$tag%') ";
          endif;
        endforeach;
	        $where .= ")
            )
            OR EXISTS (
              SELECT * FROM wp_comments
              WHERE comment_post_ID = wp_posts.ID
                AND comment_content LIKE '%$tag%'
            )
            OR EXISTS (
              SELECT * FROM wp_terms
              INNER JOIN wp_term_taxonomy
                ON wp_term_taxonomy.term_id = wp_terms.term_id
              INNER JOIN wp_term_relationships
                ON wp_term_relationships.term_taxonomy_id = wp_term_taxonomy.term_taxonomy_id
              WHERE (
          		taxonomy = 'post_tag'
            		OR taxonomy = 'category'          		
            		OR taxonomy = 'myCustomTax'
          		)
              	AND object_id = wp_posts.ID
              	AND wp_terms.name LIKE '%$tag%'
            )
        )";
    endforeach;
    return $where;
}
 
add_filter( 'posts_search', 'advanced_custom_search', 500, 2 );

searchform.php

<form method="get" id="searchform" action="<?php echo esc_url( home_url( '/' ) ); ?>">
  <input type="text" name="s" value="Enter a keyword" placeholder="Search" />
  <!-- This hidden field will default your search to be only post type you want -->    
  <input type="hidden" name="post_type" value="Your Post Type" />
  <input id="searchsubmit" type="submit" alt="Search" value="Search" />
</form>

Hey William,
I wanted to reach out and let you know that the code you provided was absolutely awesome! It worked perfectly and made a huge positive difference and performance in my WP project. Your expertise and assistance have been invaluable, and I wanted to express my heartfelt thanks. You rock!

Best regards,
Mihály | foldesistudio.hu
Hungary

@foldesistudio

Glad it helped!