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+# Global configuration
+
+All settings (see `_DEFAULTS[]` for default values) are listed here:
+
+- https://github.com/supahgreg/tt-rss/blob/main/classes/Config.php (source code, including default values)
+
+It is preferred to adjust tt-rss global configuration through the environment, i.e. defined in `.env` when using docker-compose setup:
+
+```ini
+# Copy this file to .env before building the container.
+# Put any local modifications here.
+
+TTRSS_SESSION_COOKIE_LIFETIME=2592000
+```
+
+Alternatively, you can **create** `config.php` in tt-rss root directory (copied from `config.php-dist`), using the following syntax:
+
+```js
+putenv('TTRSS_DB_HOST=myserver');
+putenv('TTRSS_SESSION_COOKIE_LIFETIME='.(86400*30));
+```
+
+- Note lack of quotes around values.
+- Options should be always prefixed by `TTRSS_`.
+- Don't modify `classes/config.php`.
+- You don't need to put everything to `config.php`, only the options which you've changed from the defaults.
+
+Legacy plugin-required constants also go to `config.php`, using `define()`:
+
+```js
+define('LEGACY_CONSTANT', 'value');
+```
+
+To set computed values via `putenv()` you have to get them evaluated by PHP, this would work:
+
+```js
+putenv('TTRSS_SESSION_COOKIE_LIFETIME='.(86400*30));
+```
+
+However, these won't give you expected results:
+
+```js
+putenv("TTRSS_SESSION_COOKIE_LIFETIME='2592000'");
+// => 0, because quoted '2592000' is an invalid number
+
+putenv('TTRSS_SESSION_COOKIE_LIFETIME=86400*30');
+// => 86400, right side expression is not evaluated,
+// instead you're casting string literal "86400*30" to an integer
+```
+
+!!! notice
+
+    All values should be precalculated when setting via `.env` because they are not evaluated by PHP and used as-is.
+
+## Minimal config.php for a non-Docker setup
+
+Should have at least these options defined:
+
+```php
+<?php
+
+putenv('TTRSS_DB_HOST=patroni.example.com');
+putenv('TTRSS_DB_USER=mydbuser');
+putenv('TTRSS_DB_PASS=mydbpass');
+putenv('TTRSS_DB_PORT=5432');
+putenv('TTRSS_SELF_URL_PATH=http://example.com/tt-rss/'); # fully-qualified URL of your tt-rss install
+putenv('TTRSS_PHP_EXECUTABLE=/path/to/php-cli-binary'); # normally something like /usr/bin/php
+```
+
+## Migrating from old-style config.php
+
+For any `config.php` settings you have changed from the defaults (normally this
+is the `DB_` group of settings and `SELF_URL_PATH`, replace as follows, using
+the rules above:
+
+`define('DB_PORT', 'xxx')` &rarr; `putenv('TTRSS_DB_PORT=xxx')`.
+
+You can safely omit any settings that were at default values.