What causes the 'unable to get local issuer certificate' error in PHP cURL?

This error typically occurs when the cURL library cannot verify the SSL certificate of the server you're trying to connect to. This is often because your PHP installation doesn't have the Certificate Authority (CA) bundle that contains the root certificate needed to validate the server's certificate chain. It can also happen if the server's certificate is self-signed or issued by a CA not trusted by your system.