add connecting to a client guide page

Author: carlos-granados

docs/getting-started/quick-start.md

@@ -42,7 +42,9 @@ The debugger connects out to your editor. Two settings decide where:
 
 Those are right when your editor and your code run on the same machine. They are
 not right from inside a container, where `localhost` is the container itself — see
-[Docker](./docker.mdx) for the setup that fixes it.
+[Docker](./docker.mdx) for the setup that fixes it, or
+[connecting to a client](../user-guide/connecting-to-a-client.md) for the rest of
+the connection settings.
 
 ## 4. Set a breakpoint
 

docs/user-guide/connecting-to-a-client.md

@@ -0,0 +1,115 @@
+---
+title: Connecting to a Client
+---
+
+The connection is made by PHP, not by your editor. Your editor waits on a port,
+and when a session starts the debugger reaches out to it and opens the connection.
+Nothing listens on the PHP side, so there is no port to expose and nothing to let
+through a firewall on the way in.
+
+So the debugger needs to know which machine and which port to reach. Two settings
+tell it, and on a normal setup both are already right:
+
+| Setting | Default | |
+| --- | --- | --- |
+| `php_debugger.client_host` | `localhost` | Where your editor is |
+| `php_debugger.client_port` | `9003` | The port it listens on |
+
+If your editor and your code run on the same machine, you are done. Change the port
+only if something else already has `9003`, or if you are debugging two projects at
+once and want them apart; whatever you pick has to match what your editor is
+listening on.
+
+Everything below is for the cases where the two are *not* on the same machine.
+
+## When the code runs somewhere else
+
+`localhost` means *this machine*, and inside a container that is the container, not
+you. The fix is to give `client_host` an address that reaches your machine from
+wherever the code runs:
+
+```ini
+php_debugger.client_host=host.docker.internal
+```
+
+See [Docker](../getting-started/docker.mdx) for the full setup, including the
+`extra_hosts` entry Linux needs to make that name resolve.
+
+### Letting the debugger work the address out
+
+Two special values stand in for an address the debugger finds at runtime. Both are
+**Linux only** — elsewhere they are ignored.
+
+```ini
+php_debugger.client_host=php_debugger://gateway
+```
+
+`gateway` connects to the gateway of the machine's default route. From inside a
+container that is the host machine, which makes it the native-Linux answer to
+`host.docker.internal` — same result, without the `extra_hosts` entry.
+
+```ini
+php_debugger.client_host=php_debugger://nameserver
+```
+
+`nameserver` uses the first DNS server the system resolver knows about, and only
+accepts it if it sits in a private range (`10/8`, `172.16/12`, `192.168/16` or
+`127/8`). A public resolver is refused. This one is for setups where your machine
+is also the one answering DNS — some VPN and corporate networks, and VMs pointed at
+the host.
+
+## When the address keeps changing
+
+On a shared or dynamic environment there may be no single address to hard-code.
+`php_debugger.discover_client_host=1` makes the debugger work it out from the
+incoming HTTP request instead, connecting back to whoever made it.
+
+The addresses it looks at, and their order, come from
+`php_debugger.client_discovery_header`, which defaults to:
+
+```ini
+php_debugger.client_discovery_header=HTTP_X_FORWARDED_FOR,REMOTE_ADDR
+```
+
+The first header present wins, and if it holds a list of addresses the first one is
+used. When no header yields an address — or the address it yields cannot be reached
+— the debugger falls back to `client_host`, so keep that set to something sensible.
+
+:::warning[Only where the request is trustworthy]
+
+`X-Forwarded-For` is a request header, and the request decides its value. On a host
+that is reachable by anyone, that lets a stranger name the address the debugger
+connects to. Use discovery behind a proxy you control, and leave it off otherwise.
+
+:::
+
+Discovery only finds the *host*. The port is always `client_port`.
+
+## How long it waits
+
+`php_debugger.connect_timeout_ms` caps each connection attempt, and defaults to
+`200` milliseconds. Every attempt that finds nothing listening costs the request
+that much, which is why the default is small.
+
+Raise it if the connection has to cross a real network and 200 ms is not enough to
+complete a handshake — a few hundred more is usually plenty.
+
+## Connecting through a relay
+
+`php_debugger.cloud_id` is for the case where the debugger cannot reach your
+machine at all — the code runs behind a firewall or a NAT you do not control, and
+no address you set would connect. Instead of dialing your editor, the debugger
+dials a hosted relay, and your editor connects to the same relay from its side.
+
+```ini
+php_debugger.cloud_id=your-id-here
+```
+
+The ID comes from the relay service, and both ends must use it. Setting it takes
+priority over `client_host` — leave it empty, as it is by default, and connections
+stay direct. Unlike the other settings on this page, it can only be set in
+`php.ini`.
+
+The relay is a third-party, paid service; see
+[its documentation](https://xdebug.org/docs/cloud) for what it costs and how to get
+an ID.

sidebars.js

@@ -33,6 +33,7 @@ const sidebars = {
       collapsed: false,
       items: [
         'user-guide/starting-the-debugger',
+        'user-guide/connecting-to-a-client',
         'user-guide/breakpoints',
         'user-guide/step-debugging',
         'user-guide/inspect-variables',