VariableSAML_IDP_ENTITY_ID
DescriptionIDP identifier used internally in the authentication flow

This identifies your IDP and is used internally in the authentication flow.

ini files
...
SAML_IDP_ENTITY_ID=$IDP_ENTITY_ID
...

The value of $IDP_ENTITY_ID could be something like, https://accounts.google.com/o/saml2?idpid=C0453.

See SAML_IDP_SIGN_ON_URL below.