How to solve this problem:
download and extract cacert.pem following the instructions at https://curl.se/docs/caextract.html
save it on your filesystem somewhere (for example, XAMPP users might use C:\xampp\php\extras\ssl\cacert.pem)
in your php.ini, put this file location in the [curl] section (putting it in the [openssl] section is also a good idea):
[curl]
curl.cainfo = "C:\xampp\php\extras\ssl\cacert.pem"
[openssl]
openssl.cafile = "C:\xampp\php\extras\ssl\cacert.pem"
restart your webserver (e.g. Apache) and PHP FPM server if applicable
(Reference: https://laracasts.com/discuss/channels/general-discussion/curl-error-60-ssl-certificate-problem-unable-to-get-local-issuer-certificate)