Assuming Windows PHP does the same as linux, you need to check the 'extra' directory that PHP will use to load extensions to php. You should see .ini files there for those extensions.
On a linux system that 'extra' diretory is /etc/php.d/ and in that directory there is a solr.ini file. It consist of one line (your error says line 0 ... that is the first line).
On linux, solr.ini file has:
extension=solr.so
Suggest you open any other .ini file in there to see what it shows in Windows.
Don't run Windows myself .... so this is an educated guess.
'spirit of sharing', Ken