Errors, Exceptions, Logging, and Debugging

PHP Error Types

Type	Meaning
Parse error	Invalid PHP syntax
Fatal error	Execution cannot continue
Warning	Problem occurred, execution may continue
Notice	Undefined variable/key or similar issue
Deprecated	API or behavior should be replaced

WordPress Debug Constants

define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);
define('SCRIPT_DEBUG', true);

Use on development or staging. Do not display errors publicly in production.

Logging

error_log('Reached checkout handler.');
error_log(print_r($settings, true));

Never log passwords, tokens, cookies, full authorization headers, or private user data.

Exceptions

try {
    myplugin_risky_operation();
} catch (Throwable $e) {
    error_log($e->getMessage());
}

Catch Throwable if you need to catch both Exception and Error.

WP_Error

Many WordPress APIs return WP_Error.

$response = wp_remote_get('https://api.example.com/items');

if (is_wp_error($response)) {
    error_log($response->get_error_message());
    return [];
}

Create a WP_Error for REST or internal failures.

return new WP_Error(
    'myplugin_invalid_request',
    __('Invalid request.', 'my-plugin'),
    ['status' => 400]
);

AJAX Error Response

wp_send_json_error([
    'message' => __('Invalid request.', 'my-plugin'),
], 400);

Debugging Checklist

• Reproduce the issue with the smallest action possible.
• Check PHP error logs.
• Confirm the hook or route actually runs.
• Confirm current user and capability.
• Confirm input after unslash and sanitization.
• Check whether the failing API returned WP_Error.
• Disable caching if the symptom is stale output.
• Test with only required plugins if conflict is suspected.

Common Problems

Symptom	Common Cause
White screen	Fatal error
Headers already sent	Output before redirect/header
Callback not running	Wrong hook or file not loaded
REST 404	Route registered too late or permalinks need flush
AJAX returns 0	Missing action or handler output issue
Form saves nothing	Nonce, capability, or name mismatch
Infinite redirect	Redirect condition never stops matching

Production Incident Rules

• Do not edit WordPress core.
• Make one change at a time.
• Preserve logs and backups.
• Disable the suspected plugin or theme safely if needed.
• Roll back code before experimenting with database changes.
• After recovery, write down the cause and prevention.