I am facing a problem in safari. i can't set third-party cookies or Session with iframe into safari. I tried p3p header but its not working. ... its working in IE
below the header scripts ....
header('P3P: CP="NOI ADM DEV COM NAV OUR STP"');
header('P3P: CP="CAO PSA OUR"');
header('P3P: CP="IDC DSP COR CURa ADMa OUR IND PHY ONL COM STA"');
header('P3P:CP="IDC DSP COR ADM DEVi TAIi PSA PSD IVAi IVDi CONi HIS OUR IND CNT"');
header('P3P: CP=HONK');
header('P3P: CP="CAO PSA OUR"');
header('P3P: CP="HONK"');
Not sure if this helps you, but I created an NPM module for setting cookies across domains. If you wanted to use JS to set/get the cross-domain cookie, this may work better.
https://www.npmjs.com/package/cookie-toss
Hope this helps!