The following warning message was displayed in a WordPress site with Avada, the best-selling WordPress theme:
Warning: array_merge(): Expected parameter 2 to be an array, null given in /home/username/public_html/wp-includes/load.php on line 66
In addition, when activating SG Optimizer of SiteGround, the following warning also appeared:
Warning: Cannot modify header information - headers already sent by (output started at /home/username/public_html/wp-includes/load.php:66) in /home/username/public_html/wp-content/plugins/sg-cachepress/core/Supercacher/Supercacher_Helper.php on line 41
The issue may be related with PHP version. In my case, the PHP version of the site was 7.3. I tried to lowered the version to PHP 7.2 or 7.1, which resolved the problems. But no problems occurred in other WordPress sites with SG Optimizer installed. I suspect there might be a WordPress plugin installed which does not support the latest PHP version. If you do not want to change PHP version, you can deactivate all plugins to see if you still get the warning messages or not.
It's easy to change the PHP version in SiteGround. To change the PHP version in SG, log into cPanel and then go to 1H Software > PHP Version Manager.
You can change the PHP version for the root folder and the first-level sub-directory:
Currently, you can specify the PHP version up to 7.3.4. The lowest version is PHP 5.6. In WordPress 5.2, the minimum PHP requirement for WP will be PHP 5.6.
In general, it's desirable to use highest version of PHP if possible. When problems occur when applying PHP 7.3, try to lower the version to 7.2 or 7.1. But avoid PHP version 5 for security and performance.
I personally prefer SiteGround, which, I think, is fast and stable but it's somewhat expensive. If you want a cheap but good performance web-hosting service, you might consider Bluehost’s Shared Hosting. If you sign up for Bluehost's Shared Hosting by clicking on this referral link, you can get one dollar off per month, starting from USD2.95/mo.
If you sign up for a service by pressing the referral links in this post, I will get some commission. However, I do not recommend specific service just for commission. I’ve been managing several WordPress sites which use different web-hosting services including SiteGround and Bluehost for more than 4 years (and this WordPress site is hosted with Bluehost.)