HI,
You need a Single Sign On solution. Both applications will need to support the same protocol ... and usually one of the application has to be the IDP (IDentity Provider). There are many technologies that can be used and OAuth2 - ala Facebook, Twitter etc seems to be the favour of the moment but there is OpenID, Shibboleth, Active Directory, SAML etc. If the other application is PHP - maybe use https://simplesamlphp.org/ to integrate the two applications. Note though the SSO happens at the time the user attempts to access resources on the other the platform and not at login. Is SSO complicated? Most of the time, yes.
You need a Single Sign On solution. Both applications will need to support the same protocol ... and usually one of the application has to be the IDP (IDentity Provider). There are many technologies that can be used and OAuth2 - ala Facebook, Twitter etc seems to be the favour of the moment but there is OpenID, Shibboleth, Active Directory, SAML etc. If the other application is PHP - maybe use https://simplesamlphp.org/ to integrate the two applications. Note though the SSO happens at the time the user attempts to access resources on the other the platform and not at login. Is SSO complicated? Most of the time, yes.