PHP Startup: Unable to load dynamic library 'C:\xampp\php\ext\php_oci8_12c.dll' - The specified procedure could not be found
On the Instant Client download page, the next line after the download link for instantclient-basic-nt-12.2.0.1.0.zip states:
The 12.2 Basic package requires the Microsoft Visual Studio 2013 Redistributable.
As you haven't listed that in what you've tried, installing it should be your next step.
You should also reboot to ensure that your changes to the environment PATH are applied system wide (e.g. to the Apache service).