Getting Started with SAML and PHP

I think that SimpleSAMLPHP is the way to go to make your application act as a full-fledged SAML 2.0 Service Provider. It's indeed a complete stack, but you can narrow down the functionalities to the bare minimum.

From my experience, configuration is a lot simpler than with Shibboleth. This step is simplified with the configuration website that ships with the distribution package.

The impact on each PHP page is relatively small: http://simplesamlphp.org/docs/1.8/simplesamlphp-sp#section_6

This is the solution we opted for on our Zend environment.

We used the PHP-SAML toolkit from OneLogin for a project that worked used non-transparant proxies (simpleSAMLphp didn't like not knowing the URL it was hosted on).

It is dead simple supporting the most basic login, but SAML 2.0 is a huge spec and it may not support what you want from it (for instance it doesn't support signing Authentication Requests).

While not a PHP specific solution, maybe Shibboleth will be an acceptable solution for you - it supports both Apache and IIS and can be used with potentially any web framework - java/php/asp.net. This also helps by being a Filter plugin into your webserver so your application will require minimal (if any) modifications to use it