wp_schedule_event function causes "Headers already sent" error

[onetarek]

I need to start the PHP session for my plugin. On plugin activation, I am getting PHP Warning.

PHP Warning:  session_start(): Cannot start session when headers already sent in..

On register_activation_hook I am using wp_schedule_event() to register my custom event hook. If I remove wp_schedule_event() I don't see the PHP Warning. So we can say that the wp_schedule_event function prints something that causes the Warning.

To test this issue use my simple plugin code below.

<?php
/**
 * Plugin Name: Test plugin
 * Description: Just a Test plugin
 * Plugin URI: http://onetarek.com
 * Author: oneTarek
 * Author URI: http://onetarek.com
 * Version: 1.0.0
 */


function test_activation() {
        wp_schedule_event( time(), 'daily', 'test_daily_event_hook' );
}

function test_deactivation() {
        wp_clear_scheduled_hook( 'test_daily_event_hook' );
}

register_activation_hook( __FILE__, 'test_activation' ); 

register_deactivation_hook( __FILE__, 'test_deactivation' );

if( session_id() == "" ){ session_start(); }

Install this plugin code. Activate and check debug log. Try activating and deactivating this plugin multiple times if you don't see any Warning.

[onetarek]

A very simple plugin to test the issue

[onetarek]

My site info

WP Version: 6.1.1
PHP version: 7.3.5 (Supports 64bit values)
Web server: Apache/2.4.43 (Unix)
Server architecture: Darwin 17.7.0 x86_64

[peterwilsoncc]

The warnings in your logs are occurring when the cron job test_daily_event_hook runs, they're not running due to the call to wp_schedule_event().

Calls to wp-cron.php send the headers early in the request. The unconditional call to start the session in your plugin will then attempt to start a session on the wp-cron.php request.

[16-Jan-2023 21:36:55 UTC] PHP Stack trace:
[16-Jan-2023 21:36:55 UTC] PHP   1. {main}() /vagrant/wordpress-develop/src/wp-cron.php:0
[16-Jan-2023 21:36:55 UTC] PHP   2. require_once() /vagrant/wordpress-develop/src/wp-cron.php:46
[16-Jan-2023 21:36:55 UTC] PHP   3. require_once() /vagrant/wordpress-develop/src/wp-load.php:50
[16-Jan-2023 21:36:55 UTC] PHP   4. include() /vagrant/wordpress-develop/src/wp-config.php:9
[16-Jan-2023 21:36:55 UTC] PHP   5. require_once() /vagrant/wp-config.php:147
[16-Jan-2023 21:36:55 UTC] PHP   6. include_once() /vagrant/wordpress-develop/src/wp-settings.php:447
[16-Jan-2023 21:36:55 UTC] PHP   7. session_start() /vagrant/content/plugins/cron-test-plugin/plugin.php:24

WP-cron doesn't support headers or sessions as it can be run either within the browser or via the CLI. I suggest you wrap the call to session_start() in a conditional to ensure it only runs in browser requests.

[onetarek]

Replying to peterwilsoncc:

The warnings in your logs are occurring when the cron job test_daily_event_hook runs, they're not running due to the call to wp_schedule_event().

Calls to wp-cron.php send the headers early in the request. The unconditional call to start the session in your plugin will then attempt to start a session on the wp-cron.php request.

[16-Jan-2023 21:36:55 UTC] PHP Stack trace:
[16-Jan-2023 21:36:55 UTC] PHP   1. {main}() /vagrant/wordpress-develop/src/wp-cron.php:0
[16-Jan-2023 21:36:55 UTC] PHP   2. require_once() /vagrant/wordpress-develop/src/wp-cron.php:46
[16-Jan-2023 21:36:55 UTC] PHP   3. require_once() /vagrant/wordpress-develop/src/wp-load.php:50
[16-Jan-2023 21:36:55 UTC] PHP   4. include() /vagrant/wordpress-develop/src/wp-config.php:9
[16-Jan-2023 21:36:55 UTC] PHP   5. require_once() /vagrant/wp-config.php:147
[16-Jan-2023 21:36:55 UTC] PHP   6. include_once() /vagrant/wordpress-develop/src/wp-settings.php:447
[16-Jan-2023 21:36:55 UTC] PHP   7. session_start() /vagrant/content/plugins/cron-test-plugin/plugin.php:24

WP-cron doesn't support headers or sessions as it can be run either within the browser or via the CLI. I suggest you wrap the call to session_start() in a conditional to ensure it only runs in browser requests.

@peterwilsoncc You are right. The header is being sent from wp-cron.php. I put my code in a condition.

<?php
if( "" == session_id() && !defined( 'DOING_CRON' ) && !isset( $_GET['doing_wp_cron'] ) ){ 
    session_start(); 
}

Now there is no warning.

[peterwilsoncc]

Thanks for following up, I appreciate the confirmation I wasn't missing an edge case.

I'll close this issue off as worksforme as it's expected behavior for wp-cron to send headers early.

You might need to change a few other things to do with sessions in your plugin but the best place to get assistance for such things is in the ​Developing with WordPress forum, the folks who hang around there are much better placed to assist.