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I backed up a Joomla 4 site and want to experiment with some changes so I uploaded it to a subdirectory and created a new database. When I ran the kickstart.php file, it ran fine. But when I clicked on the cleanup, and went to the front end, I got the following error:
syntax error, unexpected '|', expecting variable (T_VARIABLE)

I gto a 500 error when I tried to access the administrator bak end.
In the configuration.php file. I changed tthe debug to true and error reporting to maximum. THis is the result:
0

syntax error, unexpected '|', expecting variable (T_VARIABLE)
Home Page
Call stack
# Function Location
1 () JROOT/libraries/regularlabs/src/Parameters.php:27
2 Composer\Autoload\includeFile() JROOT/libraries/vendor/composer/ClassLoader.php:428
3 Composer\Autoload\ClassLoader->loadClass() JROOT/libraries/src/Autoload/ClassLoader.php:59
4 Joomla\CMS\Autoload\ClassLoader->loadClass()
5 spl_autoload_call()
6 class_exists() JROOT/plugins/system/advancedmodules/advancedmodules.php:28
7 require_once() JROOT/libraries/src/Extension/ExtensionManagerTrait.php:217
8 Joomla\CMS\Application\CMSApplication->loadPluginFromFilesystem() JROOT/libraries/src/Extension/ExtensionManagerTrait.php:160
9 Joomla\CMS\Application\CMSApplication->loadExtension() JROOT/libraries/src/Extension/ExtensionManagerTrait.php:99
10 Joomla\CMS\Application\CMSApplication->bootPlugin() JROOT/libraries/src/Plugin/PluginHelper.php:232
11 Joomla\CMS\Plugin\PluginHelper::import() JROOT/libraries/src/Plugin/PluginHelper.php:192
12 Joomla\CMS\Plugin\PluginHelper::importPlugin() JROOT/libraries/src/Application/CMSApplication.php:744
13 Joomla\CMS\Application\CMSApplication->initialiseApp() JROOT/libraries/src/Application/SiteApplication.php:635
14 Joomla\CMS\Application\SiteApplication->initialiseApp() JROOT/libraries/src/Application/SiteApplication.php:226
15 Joomla\CMS\Application\SiteApplication->doExecute() JROOT/libraries/src/Application/CMSApplication.php:293
16 Joomla\CMS\Application\CMSApplication->execute() JROOT/includes/app.php:61
17 require_once() JROOT/index.php:32

I can't determine what is causing the problem, The php version is 8.1.27 (same as the original site). I tried disabling some Regular Labs extensions:
PLG_SYSTEM_REGULARLABS plugin
PKG_REGULARLABS package pkg_regularlabs
Regular Labs Library library regularlabs
one at a time  via phpmyadmin with no success.

Can you help me find the culprit?

Thank you for your response. I didn't see anything about the php version in the htaccess file, but I tried to install it again, disabling all the Regular Labs extensions, since it was referred to in the errors. I also noticed that when I was going through the kickstart interface, it showed the existing server php at 7.1.  When I checked it in Siteground, it was set to 8.1, but I reset it anyway. Then  I refreshed the interface and it showed it as 8.1. Apparently, something was messed up. I was able to install it and enabled the Regula Labs extensions one by one and all worked without messing up the site.

So this is resolved now and I have learned to check the server php.

Thanks again for your help.

Ellen