The saml20-idp-remote.php I got from using the simplesamlphp interface, somewhere once you have the config.php and authsources.php is a way to generate the metadata. Looking at my notes I think you go to https://<XDMOD server name>/simplesaml/module.php/saml/sp/metadata.php/default-sp and it will need information from Keycloak which I got by doing this curl https://<Keycloak server name>/auth/realms/<REALM name>/protocol/saml/descriptor. If I remember correctly that will generate the PHP you need for the saml20-idp-remote.php.
Based on the errors you got it looks like your redirect URL in Keycloak is wrong, the value you should use is in the Keycloak Client JSON in the Gist linked above.