OK, so this might seem a little bit primitive, but i had to learn this the hard way.
I had the same issue, figuring that PHP doesn't see the php_solr.dll, so i thought of looking at the architecture of the PHP under the XAMPP. Assuming they were x64 as i used to install this architicture everywhere, php under XAMPP appeard as x86. There might be a mismatch with the PHP architecture with the php_solr.dll file architecture, and that sounded the only reason the .dll couldn't load for me.
Now everything is going just fine. I hope this helps.