Welcome to the MongoDB Community Forums @Jannick_Schroer !
Your error message appears to be truncated but based on certificate verify failed I expect you need to update the root certificates available in your O/S or PHP environment similar to Keep getting ServerSelectionTimeoutError - #11 by Stennie_X.
You may need help from an administrator to update certificates in a managed environment like a school web server.
MongoDB Atlas currently requires TLS 1.2 by default (see FAQ: Security for more details).
SSL3 was deprecated in 2015, but evolved into TLS 1.0 in case you are wondering about the SSL3 reference in that error message. SSL & TLS are often used interchangeably, but all modern versions of https are actually using TLS.
Regards,
Stennie