How Can I Set Cookies Across Multiple Domains for Single Sign-On (SSO) in PHP?

Cross-Domain Cookie Conundrum

In the realm of web development, a common challenge arises when attempting to set cookies across multiple domains. While security concerns rightly restrict this action, there are scenarios where it becomes necessary, such as Single Sign-On (SSO).

The Question

The dilemma stems from the need for a domain, let's call it "account.domain.com," to set a cookie that indicates a user's logged-in status for a group of domains, for example:

• domain.com
• domain1.com
• domain2.com

The Answer

Regrettably, there is no straightforward solution using PHP and cookies to achieve cross-domain cookie setting. The fundamental issue is that each domain maintains its own cookie storage, preventing setting cookies from one origin on another.

Alternatives

To circumvent this limitation, alternative approaches exist:

1. Back Channel: Domains establish direct communication channels to exchange user identity and session status information.
2. Token Passing: When a user's browser navigates between domains, a digitally signed token is passed in the request parameters. This token carries the user's identity and session information, allowing each domain to independently verify the user's status.

Implementing Single Sign-On

Implementing SSO in this scenario requires careful planning and the use of specialized tools or frameworks. SimpleSAMLPHP is a PHP implementation that facilitates SSO by providing secure token handling and communication between domains. It eliminates the need to roll out your own custom solution.

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