$_GET empty

Are you using the  <?php ?> tag when you set up the contextual filter in Views → Contextual Filters (default value / validation criteria)? 

It should not be used, according to the directions there.

In my testing, this is working just fine. This is what I'm doing:

1. Edit the view node_admin_content at admin/structure/views/view/node_admin_content
2. Add a contextual filter for nid
3. Select "Provide default value"
4. Enter this (obviously just for this test, it will never work in a different context)

debug($_GET['title']);
return 1;

5. Save everything. PLEASE NOTICE: print doesn't seem  to work in this context (the print output gets flushed out when building the output). So, you must use debug() to output this value!

6. Visit admin/content/node?title=first&type=All&status=All in a clean install of BAckdrop

You get this:

As for the PHP filter module - this may be a separate problem. PHP filter doesn't have a release in BAckdrop and will not have one, and the last merge was done 3 years ago. There are ways to avoid using PHP filter. If you post here about your intended use, someone may be able to help.

Ok, glad to hear it works for you. Can you mark your answer as "correct" and also mine as "helpful"? This will help people search for answers in the future.

May I append a question here?

I seem to have a similar issue:
Two Backdrop 1.30.1 portals, both running on the same server (Apache, PHP, MariaDB), both with the same enabled modules and theme.
Both have the same Apache confs, and - as far as I can see - the same settings in the .htaccess file for rewrite rules.

Difference:

• One Backdrop 1.30.1 portal is a completely fresh installation.
• The other Backdrop 1.30.1 portal is an upgrade from Drupal 7, which works fine and shows no error messages. Of course, settings and content have been taken over from the Drupal 7 portal.

No matter what I try, $_GET

• works in the fresh installation
• does not work in the Drupal 7 upgrade—no URL parameters are returned at all:
  $_GET['parameter'] is empty
  $_GET only shows Array ( [q] => node/100 )

Maintenance mode is off.
I have tested this on pages with the PHP filter, as well as with Views cont. filters, etc.

Where could the problem be? Something with the redirection on the Drupal Update?

===

Enabled modules on both portals
============================

Core
----------
admin_bar                    1.30.1
block                        1.30.1
color                        1.30.1
comment                      1.30.1
config                       1.30.1
contextual                   1.30.1
date                         1.30.1
dblog                        1.30.1
entity                       1.30.1
field                        1.30.1
field_sql_storage            1.30.1
field_ui                     1.30.1
file                         1.30.1
filter                       1.30.1
image                        1.30.1
language                     1.30.1
layout                       1.30.1
list                         1.30.1
locale                       1.30.1
menu                         1.30.1
node                         1.30.1
number                       1.30.1
options                      1.30.1
path                         1.30.1
search                       1.30.1
system                       1.30.1
taxonomy                     1.30.1
text                         1.30.1
translation                  1.30.1
update                       1.30.1
user                         1.30.1
views                        1.30.1
views_ui                     1.30.1

Contrib
-------
addressfield                 1.x-1.3.2
clone                        1.x-1.1.6
colorbox                     1.x-2.17.2
entity_plus                  1.x-1.0.21
geocoder                     1.x-1.0.2
geofield                     1.x-1.0.4
globalredirect               1.x-1.6.0
honeypot                     1.x-1.26.1
leaflet                      1.x-1.2.0
metatag                      1.x-1.22.6
metatag_views                1.x-1.22.6
print                        1.x-2.4.0-beta6
print_ui                     1.x-2.4.0-beta6
views_bulk_operations        1.x-3.0.6
webform                      1.x-4.26.3
webform_validation           1.x-1.16

After thoroughly comparing the entire UI configuration, .htaccess, and /files/configxx/active/*.json without finding any differences, I decided to restart Apache multiple times and repeatedly clear all caches under /admin/config/development/performance.

Until now, I had only clicked "Clear all caches" once.

After clearing the caches about 10 times in a row, $_GET has slowly started working again on some subpages. Not all of them yet, but let's see how it develops.