Twitter API returns NULL on XAMPP

Like Jimbo pointed out in the comments above, the solution can be found here

According to the article, the Windows PHP distribution doesn't come with an up-to-date bundle of the CA root certificates.However, the bundle can be downloaded here. I put it under C:\xampp\php\cacert.pem.

The second step is to add curl.cainfo=c:\xampp\php\cacert.pem at the end of your php.ini file.Change the path to your folder where you saved the .pem-file.