CodeIgniter Hooks: Minimal Architecture for Safe Lifecycle Injection

CodeIgniter 3 Hooks let you run custom code at fixed points in the request lifecycle without editing core files. The useful takeaway is that they are in-process, trusted extensions, not a plugin sandbox. Use them for cross-cutting, admin-controlled behavior only, and keep the design minimal so a failure cannot take down the request before a controller runs.

Requirements for Hooks in CodeIgniter 3

This note assumes CodeIgniter 3.x. CodeIgniter 4 replaces Hooks with Events.

Hooks require two configuration changes and a file on disk.

• Set $config['enable_hooks'] = TRUE; in application/config/config.php. Without this the hooks.php file is ignored.
• Define points in application/config/hooks.php. Each point is an array with class, function, filename and filepath.
• The hook file must be readable by the web server user and free of PHP syntax errors. Hooks run inside the same PHP process as the request.

Smallest suitable design

A minimal, safe hook uses one point, one file, and a plain function. No class loading is required.

In application/config/hooks.php:

$hook['pre_controller'][] = array(
  'class'    => '',
  'function' => 'log_request',
  'filename' => 'request_logger.php',
  'filepath' => 'hooks'
);

In application/hooks/request_logger.php:

<?php
function log_request()
{
    log_message('debug', 'Hook executed at '.microtime(true));
}

Place the file under application/hooks/. Permissions needed are write access to config files to edit them and read access for the hook file. Risk: a PHP parse error in the hook file produces a fatal error before routing.

Rollback is immediate: set $config['enable_hooks'] = FALSE; or remove the entry from hooks.php to stop execution.

Trust and data boundaries

Hooks execute with full privileges of the request. They can read superglobals, access the CodeIgniter superobject via get_instance(), and load libraries.

Because of this, treat hooks as trusted code only. Loading a hook from an untrusted source is remote code execution. Do not build a system where users can supply hook filenames or function names.

Data boundary: a hook can mutate global state, session data, and output buffers. Changes made in pre_system affect everything that follows; changes in post_system run after output is sent.

Operational checks and verification

Verify enablement and execution without assuming success.

1. Ensure logging is on in application/config/config.php so log_message writes to application/logs/.
2. Load a page and inspect application/logs/*.log for the debug line added by the hook.
3. Alternatively, register a post_controller_constructor hook that appends a known string to output and confirm the string appears in the rendered page.

Operational checks to keep in place: file readability, no syntax errors, and monitoring log size. Each hook adds overhead; a large number of heavy hooks increases request latency.

Failure modes

A thrown exception or fatal error inside a hook aborts the request before controllers run if the point is early, such as pre_system or pre_controller.

A misspelled hook point name is silently ignored. CodeIgniter logs hook-related errors to its log file, but it does not raise an alert for a missing point.

Hooks that modify core objects or the output buffer can introduce side effects that break later framework steps. Because hooks share memory, ordering matters when multiple hooks register for the same point.

When to change the design

Move away from Hooks if requirements change.

• Untrusted user code: Hooks cannot be sandboxed. Use a separate plugin system with restricted APIs instead of Hooks.
• Zero-overhead requirement: Remove hooks entirely or gate them behind environment checks. Hooks always incur load and execution cost.
• Post-output modification needed: Use post_system for shutdown work. Earlier points cannot affect headers already sent.

Limitations: Hooks are CodeIgniter 3 only, run synchronously in the request path, and provide no isolation. Keep them small, observable via logs, and owned by the deployment team.