Easy way to process search results before displaying

Question

I've got several search forms on my site and I want them to show different results. My website has a very strict hierarchy and the search form on a parent site should only show results from it's child pages.

My plan was to include different hidden fields on the different parent pages which contain the id of that particular page. In the search.php I then wanted to process the results and filter out the pages and posts that have no relation to the parent page.

Is there an easy way on how to achieve this?

Thanks in advance.

EDIT 1

---

This is my search.php

<?php 
if (have_posts()){
  while(have_posts()){
    the_post(); ?>
    <div>
      <h4><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h4>
      <p><?php echo get_the_author(); ?> - <?php echo get_the_date(); ?></p>
      <p><?php echo get_the_excerpt(); ?></p>
    </div>
    <?php
  }
} else{ ?>
  <h3>Sorry</h3>
  <p>We are sorry but we could not find any matching articles on our site. Please try again with an other search request.</p>
  <?php
    get_search_form ();
}
?>

And the searchform.php:

<form role="search" method="get" id="searchform" action="<?php echo esc_url( home_url( '/' ) ); ?>">
  <label>Search...</label>
  <input type="text" name="s" id="s" value="<?php echo get_search_query(); ?>" placeholder="Search..." />
  <input type="hidden" name="post_parent" value="<?php echo (int)get_the_ID(); ?>" />
  <button type="submit"><i class="fa fa-search" aria-hidden="true"></i></button>
</form>

EDIT 2

I also added <?php wp_reset_query(); ?> before the if(have_posts()){}. This results in no change. The pages are still shown.

Answer 1

You can use pre_get_posts filter to filter out what you need. There's an example on how to do this in Codex:

https://codex.wordpress.org/Plugin_API/Action_Reference/pre_get_posts#Exclude_Pages_from_Search_Results

function search_filter($query) {
  if ( !is_admin() && $query->is_main_query() ) {
    if ($query->is_search) {
      $query->set('post_type', 'post');
    }
  }
}

add_action('pre_get_posts','search_filter');

Also, this article might get you in the direction of editing the search form...

Answer 2

While the pre_get_posts filter is going to let you modify the main query before it happens, letting you add extra requirements, in this case it isn't necessary! You can do it all with the URL and the search form

First, the query variables passed into WP_Query can be used in the URL. post_parent being the query var you're wanting.

So if we have this:

<form action="/" method="get">
    <input type="text" name="s" />
    <input type="hidden" name="post_parent" value="<?php echo (int)get_the_ID(); ?>"/>
    <input type="hidden" name="post_type" value="page"/>
</form>

Then any search results will be limited to those whose post_parent is the page that the search came from. You'll see URLs such as /?s=test&post_parent=123.

This trick can be used to filter by categories and authors, for example, searching a category:

/category/example/?s=test

Only showing posts by a particular author in a date archive:

/2016/?author=123