Xdebug3 does not trigger

I'm trying to set up Xdebug for a remote web server. The idea is to use ssh tunnels from the development machines to the development server. The system is a Debian Bookworm LXC container. The network part apparently works:

# establish ssh to web server
dev@client:~$ ssh -R host.example.com:9003:localhost:9000 site
site@host:~$ php -i | grep xdebug
# has lots of settings of course, among those:
xdebug.client_host => localhost => localhost
xdebug.client_port => 9003 => 9003
# fire up dbgpClient on client
dev@client:~$ dbgpClient -p 9000
# test for connection
site@host:~$ telnet localhost 9003
# yields Connect from [::1]:54680 in dbgpClient

So, connecting to port 9003 on the web server is seen by dbgpClient on port 9000 on the development client. But PHP does not trigger the client:

# fire up dbgpClient on client
dev@client:~$ dbgpClient -p 9000
# run interactive PHP on server
site@host:~$ export XDEBUG_SESSION=1
site@host:~$ php -a -dxdebug.start_with_request=yes -dxdebug.log=/tmp/xdebug.log
Interactive shell
php > echo 1;
1
php > ^C
site@host:~$ cat /tmp/xdebug.log
site@host:~$

And of course no connection attempt in dbgpClient. Using a simple test script instead of an interactive shell yields the same result. After running the test, telnet still connects as before.

I read through the myriads of configuration options for Xdebug, but I could not find anything conclusive. But I must admit that I hardly understand a quarter of the options, and it's the first time ever I try to set up Xdebug-3. So, for the wizards - this is the complete php -i | grep xdebug:

/etc/php/8.2/cli/conf.d/20-xdebug.ini,
xdebug
Support Xdebug on Patreon, GitHub, or as a business: https://xdebug.org/support
Enabled Features (through 'xdebug.mode' setting)             
'xdebug://gateway' pseudo-host support => yes
'xdebug://nameserver' pseudo-host support => yes
xdebug.auto_trace => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.cli_color => 0 => 0
xdebug.client_discovery_header => HTTP_X_FORWARDED_FOR,REMOTE_ADDR => HTTP_X_FORWARDED_FOR,REMOTE_ADDR
xdebug.client_host => localhost => localhost
xdebug.client_port => 9003 => 9003
xdebug.cloud_id => no value => no value
xdebug.collect_assignments => Off => Off
xdebug.collect_includes => (setting removed in Xdebug 3) => (setting removed in Xdebug 3)
xdebug.collect_params => (setting removed in Xdebug 3) => (setting removed in Xdebug 3)
xdebug.collect_return => Off => Off
xdebug.collect_vars => (setting removed in Xdebug 3) => (setting removed in Xdebug 3)
xdebug.connect_timeout_ms => 200 => 200
xdebug.coverage_enable => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.default_enable => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.discover_client_host => Off => Off
xdebug.dump.COOKIE => no value => no value
xdebug.dump.ENV => no value => no value
xdebug.dump.FILES => no value => no value
xdebug.dump.GET => no value => no value
xdebug.dump.POST => no value => no value
xdebug.dump.REQUEST => no value => no value
xdebug.dump.SERVER => no value => no value
xdebug.dump.SESSION => no value => no value
xdebug.dump_globals => On => On
xdebug.dump_once => On => On
xdebug.dump_undefined => Off => Off
xdebug.file_link_format => no value => no value
xdebug.filename_format => no value => no value
xdebug.force_display_errors => Off => Off
xdebug.force_error_reporting => 0 => 0
xdebug.gc_stats_enable => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.gc_stats_output_dir => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.gc_stats_output_name => gcstats.%p => gcstats.%p
xdebug.halt_level => 0 => 0
xdebug.idekey => no value => no value
xdebug.log => no value => no value
xdebug.log_level => 7 => 7
xdebug.max_nesting_level => 256 => 256
xdebug.max_stack_frames => -1 => -1
xdebug.mode => develop => develop
xdebug.output_dir => /tmp => /tmp
xdebug.overload_var_dump => (setting removed in Xdebug 3) => (setting removed in Xdebug 3)
xdebug.profiler_append => Off => Off
xdebug.profiler_enable => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.profiler_enable_trigger => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.profiler_enable_trigger_value => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.profiler_output_dir => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.profiler_output_name => cachegrind.out.%p => cachegrind.out.%p
xdebug.remote_autostart => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.remote_connect_back => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.remote_enable => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.remote_host => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.remote_log => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.remote_log_level => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.remote_mode => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.remote_port => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.remote_timeout => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.scream => Off => Off
xdebug.show_error_trace => Off => Off
xdebug.show_exception_trace => Off => Off
xdebug.show_local_vars => Off => Off
xdebug.show_mem_delta => (setting removed in Xdebug 3) => (setting removed in Xdebug 3)
xdebug.start_upon_error => default => default
xdebug.start_with_request => default => default
xdebug.trace_enable_trigger => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.trace_enable_trigger_value => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.trace_format => 0 => 0
xdebug.trace_options => 0 => 0
xdebug.trace_output_dir => (setting renamed in Xdebug 3) => (setting renamed in Xdebug 3)
xdebug.trace_output_name => trace.%c => trace.%c
xdebug.trigger_value => no value => no value
xdebug.use_compression => 0 => 0
xdebug.var_display_max_children => 128 => 128
xdebug.var_display_max_data => 512 => 512
xdebug.var_display_max_depth => 3 => 3

What else can I do to zero in on the issue?

Solution

Xdebug does not trigger because you haven't enabled the debugger.

In your phpinfo() output it shows:

xdebug.mode => develop => develop

That needs to be set to debug, or develop,debug. You can do that on the command line in multiple ways:

set also -dxdebug.mode=develop,debug
add the xdebug.mode=develop,debug to your php.ini file (or 90-xdebug.ini file, if you have one)
set the XDEBUG_MODE=debug,develop environment variable on the command line, just like you set XDEBUG_SESSION=1