Dolphin 7.4.2 works fie with PHP 7, but won't work on version 8. I've installed 7.4.2 dozens of times and it can easily be done in less than 10 minutes. The error may not be 7.4.2, or PHP, but it could be the ini file settings which vary from server to server. Hosted servers are a pain because they must consider others using the same version of PHP. VPS servers are much better because you're in charge of PHP. If you're using a hosted server, you may need to contact your hosting service.
Make sure PHP 7.x is installed and make sure you're installing Dophin 7.4.2 which should be the latest download.