Adding a hook to ‘parse_request’ so that siteurl/pagename/xyz ignores “xyz”

I have a unique project that I am trying to prototype where some of a page’s assets comes from wordpress, whereas others come in from a request to a remote API.

Currently I use a page called “series” that uses this structure to make a wp_get_remote call to pass data into the page:

site_url/series/?pname=xyz

And would like to change it to be pretty:
site_url/series/xyz

So the effect would be the series page is called, and then my request runs to call in the data I need from the endpoint. I need to run this as soon as possible, because the endpoint has data that I would like to set to the meta description, plus the majority of the page content relies on the data I retrieve.

I have been trying to use the hook ‘parse_request’, but I get a Warning: Cannot modify header information headers already sent by .. pluggable.php

add_action( 'parse_request', 'test_changes' );

function test_changes( $query ) {

$request_uri_string = $_SERVER['REQUEST_URI'];

echo $request_uri_string;
echo "<br>";

if( strpos($request_uri_string, 'series') !== false ){   
    echo "theres a series here";
    unset($query->query_vars);
    $query->query_vars[ 'pagename' ] = "series";
    $query->query_vars[ 'page' ] = "";
    $query->request = "series";
    $query->matched_query = "pagename=series&page=";
} 
//See what is returned.
echo "<br><br><p>Parse Request</p>";
var_dump($query);

return $query;
}

Trying to get some insight into why this doesn’t work, solutions to create the result I want, and perhaps some other ways you can use ‘parse_request’.

I would just add a rewrite rule rather than try to modify query parsing.

function wpd_series_query_var( $vars ){
    $vars[] = 'wpd_series';
    return $vars;
}
add_filter( 'query_vars', 'wpd_series_query_var' );

function wpd_series_rewrite_rule() {
    add_rewrite_rule(
        '^series/([^/]+)/?$',
        'index.php?pagename=series&wpd_series=$matches[1]',
        'top'
    );
}
add_action( 'init', 'wpd_series_rewrite_rule' );

Don’t forget to flush_rewrite_rules() after adding new ones.

You can then get wpd_series anywhere after the parse_query action with get_query_var('wpd_series').