Redirect a WP created page to a manually created PHP file inside theme folder

Maybe you can use query_vars filter and parse_request action.

add_action('init', 'initInternalWPRewrite', 10);
// Add your rewrite rule, you don't need to create a wordpress page
function initInternalWPRewrite () {
    add_rewrite_rule('^diagnostics/?$', 'index.php?diagnostics=1', 'top');
}

add_action('init', 'rewriteRulesCacheInit', 10);
// This method is useful to avoid manual flush rewrite cache
function rewriteRulesCacheInit () {
    if (!get_option('rewrite_rules_cache')) {
        add_option('rewrite_rules_cache', true);
    }
}
// With this method you don't need to flush cache
add_action('init', 'rewriteRulesFlushCache', 20);
function rewriteRulesFlushCache () {
    if (get_option('rewrite_rules_cache')) {
        flush_rewrite_rules();
        delete_option('rewrite_rules_cache');
    }
}
// Add your custom query var
add_filter('query_vars', 'queryVars');
function queryVars ($query_vars) {
    $query_vars[] = 'diagnostics';

    return $query_vars;
}

// Render your PHP content, please, avoid include PHP or if you want to include PHP file, ensure is defined ABSPATH constant to avoid direct execution
// REMEMBER: exit or die, if you don't do this, WordPress will be loaded.
add_action('parse_request', 'renderDiagnosticPage');
function renderDiagnosticPage ($wp_query) {
    if (array_key_exists('diagnostics', $wp_query->query_vars)) {
        //your code
        exit();
    }
    return;
}

More info:

• Query Vars filter: https://codex.wordpress.org/Plugin_API/Filter_Reference/query_vars

• Parse Request action: https://codex.wordpress.org/Plugin_API/Action_Reference/parse_request

  • add_rewrite_rule method: https://codex.wordpress.org/Rewrite_API/add_rewrite_rule

Hope this helps you.