Search within selected category in WordPress - php

Question

I need to alter the search behavior of a WordPress site where the theme doesn't use searchform.php at all (it's the Divi theme). I'd rather not employ a plugin to do this as the site has many plugins already.
The default state of the page is to show "all posts from all categories" (/?cat=0) with a category list dropdown generated using wp_dropdown_categories() that allows the user to select other categories.
The desired use case is:
The user selects a category from a dropdown
The browser goes to the selected category's archive
The user types a search query into the search box on the category archive with the intent of seeing only posts and pages in that category that contain the search term.
I figured I could use the pre_get_posts hook to do this. So I did the following:
function search_by_cat()
{
global $wp_query;
if ( is_search() ) {
$currenturl = add_query_arg( $wp->query_vars, home_url( $wp->request ) );
$previousurl = wp_get_referer();
if ( strpos( $previousurl, '/category/' ) !==false ) {
$searchcategoryurl = untrailingslashit( $previousurl ) . $currenturl;
wp_redirect( $searchcategoryurl );
exit();
}
}
}
add_action('pre_get_posts', 'search_by_cat');
But this just results in ERR_TOO_MANY_REDIRECTS in the browser. I think it might be because the user is submitting a search that results in the same destination as the referrer. How do I break out of the redirect loop so I can (essentially) redisplay the page of results? Or is my entire approach all wrong? Is there a better way?

Answer 1

Here's the general idea from my comments, you'll need to change the CPT and taxonomy types to match your code. Tax queries are a little weird because of the nested array syntax. The comments should hopefully help understand it more.
add_action(
'pre_get_posts',
static function (WP_Query $query) {
// We only want to run on the main search query
if (!$query->is_search() || !$query->is_main_query()) {
return;
}
// Attempt to get the parameter, or default to 0
$category_id = (int)($_GET['category_id'] ?? 0);
if (!$category_id) {
return;
}
// Since we're search by a taxonomy, we probably want to bind it to a type, too
$query
->set(
'post_type',
// Change as needed
'events'
);
$query
->set(
'tax_query',
[
// See this for specifics: https://developer.wordpress.org/reference/classes/wp_query/#taxonomy-parameters
[
// Change as needed
'taxonomy' => 'event-type',
'field' => 'term_id',
'terms' => [$category_id],
],
]
);
}
);