multiple domain session sharing

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I want to share the session between www.abc.com and login.abc.com.

i've used this code. i can't edit php.ini directly as my web host wont allow it.

<?php ini_set("session.cookie_domain", ".abc.com"); session_start();?>

it seems to work fine for a day.

today everything is a mess. when a user log in in login.abc.com and click on a link (to www.abc.com) or open a new www.abc.com page, login.abc.com will log out automatically. it will not pass the session to www.abc.com at all.

i've looked and googled for solutions on multiple domain sharing. but not even one worked for me.

i've also tried using this codes.

<?php
session_set_cookie_params(0 , '/', '.abc.com', time()+3600);
session_start();
?>

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suddenly now it works. weird. i've emailed the web hosting company. they said they didn't make any changes.

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nope.

this time login.abc.com wont log out automatically when i clicked on a link to www.abc.com.

the session remains.

but still not passed to www.abc.com.

i created a new sub domain and copied all www.abc.com files there.

it seems to work fine! does setting directory permission has something to do with blocking sessions? because i didnt touch the permission at all for the new subdomain. permission like directory browsing etc...

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my theory is the session doesnt work for the main domain (www.abc.com). it work well with its all subdomain (login.abc.com, test.abc.com).

can someone explain to me why...