Symptoms
When trying to access WordPress wp-admin login to manage the WordPress site you see Error 500 and the PHP error_log contains
PHP Fatal error: Uncaught Error: [] operator not supported for strings in /home/archbtwu/public_html/wp-content/plugins/revslider/includes/framework/base-admin.class.php:71
Stack trace:
Description
This issue is seen when a plugin causes an issue and likely isn't compatible with the present PHP version. In the above case the revslider plugin is causing issues when PHP 7.0 or higher was set but was able to be loaded when PHP 5.6 was set.
Workaround
There are two options. The first is to set the PHP version to PHP 5.6 in this case and then upgrade the plugin or leave it on the current PHP version and rename the plugin so wp-admin can be accessed and the plugin reinstalled if possible.
This is done in the above case with the revslider plugin causing the issue
cd /home/user/path/to/documentroot/
mv -v wp-content/plugins/revslider{,.OLD}