wp-plugin-development

# WP Plugin Development

## When to use

Use this skill for plugin work such as:

- creating or refactoring plugin structure (bootstrap, includes, namespaces/classes)
- adding hooks/actions/filters
- activation/deactivation/uninstall behavior and migrations
- adding settings pages / options / admin UI (Settings API)
- security fixes (nonces, capabilities, sanitization/escaping, SQL safety)
- packaging a release (build artifacts, readme, assets)

## Inputs required

- Repo root + target plugin(s) (path to plugin main file if known).
- Where this plugin runs: single site vs multisite; WP.com conventions if applicable.
- Target WordPress + PHP versions (affects available APIs and placeholder support in `$wpdb->prepare()`).

## Procedure

### 0) Triage and locate plugin entrypoints

1. Run triage:
   - `node skills/wp-project-triage/scripts/detect_wp_project.mjs`
2. Detect plugin headers (deterministic scan):
   - `node skills/wp-plugin-development/scripts/detect_plugins.mjs`

If this is a full site repo, pick the specific plugin under `wp-content/plugins/` or `mu-plugins/` before changing code.

### 1) Follow a predictable architecture

Guidelines:

- Keep a single bootstrap (main plugin file with header).
- Avoid heavy side effects at file load time; load on hooks.
- Prefer a dedicated loader/class to register hooks.
- Keep admin-only code behind `is_admin()` (or admin hooks) to reduce frontend overhead.
- Add `declare(strict_types=1)` at the top of every PHP file.
- Follow naming conventions: snake_case for variables/functions/methods, PascalCase for classes, UPPER_SNAKE_CASE for constants.

See:
- `references/structure.md`

### 1a) PHP code style

- Require `declare(strict_types=1)` at the top of every file.
- Follow naming conventions (see reference).
- Use Heredoc/Nowdoc for all multi-line HTML output; never use `.` string concatenation for HTML.
- Class structure: properties → constructor → register_hooks → public → private.
- Default to static methods with `register_hooks()` for hook registration.

See:
- `references/php-code-style.md`

### 2) Hooks and lifecycle (activation/deactivation/uninstall)

Activation hooks are fragile; follow guardrails:

- register activation/deactivation hooks at top-level, not inside other hooks
- flush rewrite rules only when needed and only after registering CPTs/rules
- uninstall should be explicit and safe (`uninstall.php` or `register_uninstall_hook`)

See:
- `references/lifecycle.md`

### 3) Settings and admin UI (Settings API)

Prefer Settings API for options:

- `register_setting()`, `add_settings_section()`, `add_settings_field()`
- sanitize via `sanitize_callback`

See:
- `references/settings-api.md`

### 4) Security baseline (always)

Before shipping:

- Validate/sanitize input early; escape output late.
- Use nonces to prevent CSRF *and* capability checks for authorization.
- Avoid directly trusting `$_POST` / `$_GET`; use `wp_unslash()` and specific keys.
- Use `$wpdb->prepare()` for SQL; avoid building SQL with string concatenation.

See:
- `references/security.md`

### 5) Data storage, cron, migrations (if needed)

- Prefer options for small config; custom tables only if necessary.
- For cron tasks, ensure idempotency and provide manual run paths (WP-CLI or admin).
- For schema changes, write upgrade routines and store schema version.

See:
- `references/data-and-cron.md`

## Verification

- Plugin activates with no fatals/notices.
- Settings save and read correctly (capability + nonce enforced).
- Uninstall removes intended data (and nothing else).
- Run repo lint/tests (PHPUnit/PHPCS if present) and any JS build steps if the plugin ships assets.
- Run `composer lint` (phpcs) and `composer analyse` (phpstan) if available.

## Failure modes / debugging

- Activation hook not firing:
  - hook registered incorrectly (not in main file scope), wrong main file path, or plugin is network-activated
- Settings not saving:
  - settings not registered, wrong option group, missing capability, nonce failure
- Security regressions:
  - nonce present but missing capability checks; or sanitized input not escaped on output

See:
- `references/debugging.md`

## Escalation

For canonical detail, consult the Plugin Handbook and security guidelines before inventing patterns.