Plugin: Document the project structure and conventions used in the `l…

…ib` folder for PHP code (WordPress#39603)

* Add lib/README.md

* Subediting

* Formatting

* Fix typo

Co-authored-by: Daniel Richards <daniel.richards@automattic.com>

* Clarify why we want the wp_ prefix

* Reword sentence about hooking

* Fix typo

* Clarify that gutenberg_ is acceptable for infrastrucutre

Co-authored-by: Ramon <ramonjd@users.noreply.github.com>
Co-authored-by: Daniel Richards <daniel.richards@automattic.com>

lib/README.md

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+# Gutenberg PHP
+
+## File structure
+
+Gutenberg adds features to WordPress Core using PHP hooks and filters. Some
+features, once considered stable and useful, are merged into Core during a Core
+release. Some features remain in the plugin forever or are removed.
+
+To make it easier for contributors to know which features need to be merged to
+Core and which features can be deleted, Gutenberg uses the following file
+structure for its PHP code:
+
+- `lib/experimental` - Experimental features that exist only in the plugin. They
+  are not ready to be merged to Core.
+- `lib/stable` - Stable features that exist only in the plugin. They could one
+  day be merged to Core, but not yet.
+- `lib/compat/wordpress-X.Y` - Stable features that are intended to be merged to
+  Core in the future X.Y release, or that were previously merged to Core in the
+  X.Y release and remain in the plugin for backwards compatibility when running
+  the plugin on older versions of WordPress.
+
+## Best practices
+
+### Prefer the `wp` prefix
+
+For features that may be merged to Core, it's best to use a `wp_` prefix for functions or a `WP_` prefix for classes. 
+
+This applies to both experimental and stable features.
+
+Using the `wp_` prefix avoids us having to rename functions and classes from `gutenberg_` to `wp_` if the feature is merged to Core.
+
+Functions that are intended solely for the plugin, e.g., plugin infrastructure, should use the `gutenberg_` prefix.
+
+#### Feature that might be merged to Core
+
+```php
+function wp_get_navigation( $slug ) { ... }
+```
+
+#### Plugin infrastructure that will never be merged to Core
+
+```php
+function gutenberg_get_navigation( $slug ) { ... }
+```
+
+### Group PHP code by _feature_
+
+Developers should organize PHP into files or folders by _feature_, not by _component_.
+
+When defining a function that will be hooked, developers should call `add_action` and `add_filter` immediately after the function declaration.
+
+These two practices make it easier for PHP code to start in one folder (e.g., `lib/experimental`) and eventually move to another using a simple `git mv`.
+
+#### Good
+
+```php
+// lib/experimental/navigation.php
+
+function wp_get_navigation( $slug ) { ... }
+
+function wp_register_navigation_cpt() { ... }
+
+add_action( 'init', 'wp_register_navigation_cpt' );
+```
+
+#### Not so good
+
+```php
+// lib/experimental/functions.php
+
+function wp_get_navigation( $slug ) { ... }
+
+// lib/experimental/post-types.php
+
+function wp_register_navigation_cpt() { ... }
+
+// lib/experimental/init.php
+add_action( 'init', 'wp_register_navigation_cpt' );
+```
+
+### Wrap functions and classes with `! function_exists` and `! class_exists`
+
+Developers should take care to not define functions and classes that are already defined.
+
+When writing new functions and classes, it's good practice to use `! function_exists` and `! class_exists`.
+
+If Core has defined a symbol once and then Gutenberg defines it a second time, fatal errors will occur.
+
+Wrapping functions and classes avoids such errors if the feature is merged to Core. 
+
+#### Good
+
+```php
+// lib/experimental/navigation/navigation.php
+
+if ( ! function_exists( 'wp_get_navigation' ) ) {
+	function wp_get_navigation( $slug ) { ... }
+}
+
+// lib/experimental/navigation/class-wp-navigation.php
+
+if ( class_exists( 'WP_Navigation' ) ) {
+	return;
+}
+
+class WP_Navigation { ... }
+```
+
+#### Not so good
+
+```php
+// lib/experimental/navigation/navigation.php
+
+function wp_get_navigation( $slug ) { ... }
+
+// lib/experimental/navigation/class-gutenberg-navigation.php
+
+class WP_Navigation { ... }
+```
+
+Furthermore, a quick codebase search will also help you know if your new method is unique.
+
+### Note how your feature should look when merged to Core
+
+Developers should write a brief note about how their feature should be merged to Core, for example, which Core file or function should be patched. 
+
+Notes can be included in the doc comment.
+
+This helps future developers know what to do when merging Gutenberg features into Core.
+
+#### Good
+
+```php
+/**
+ * Returns a navigation object for the given slug.
+ * 
+ * Should live in `wp-includes/navigation.php` when merged to Core.
+ *
+ * @param string $slug
+ *
+ * @return WP_Navigation
+ */
+function wp_get_navigation( $slug ) { ... }
+```