Domain name and cookie issues (domain name suffix)_PHP tutorial

Domain name and cookie
I accidentally thought of a question: www.g.cn can set the cookie to .g.cn, so can www.com.cn set the cookie to .com.cn?

Test result: No. Because the browser knows that the suffix of www.com.cn is .com.cn instead of .cn, it prohibits setting cookies.
Because the browser has a built-in domain name suffix list. todo: If a new suffix appears in the future and the old browser cannot update the list, will it allow cookies to be set?

	extension后缀	一级域名	二级域名
www.g.cn	.cn	g.cn	*.g.cn
www.com.cn	.com.cn	www.com.cn	*.www.com.cn
www.google.com.cn	.com.cn	google.com.cn	*.google.com.cn

Can www.example.com read the cookie of .example.com?
Yes.
Can www.example.com read the cookie of example.com?
No. todo: Use SSO for www.example.com and example.com to prevent cookies from being brought to static.example.com.
Can example.com read the cookie of www.example.com?
Answer: No.
setcookie('a', 'aa', time() + 1234, '/', 'example.com'); Is the cookie set to .example.com or example.com?
Answer: It’s from .example.com.
If you want to set a cookie for example.com, you need to use setcookie('default', 'default', time() + 1234, '/');.
Cookie setting and reading range:

HTTP请求域名	一级域名	cookie可设置（并可读取）的范围	cookie不可设置	cookie不可读取
example.com	example.com	example.com，.example.com	www.example.com	www.example.com
www.example.com	example.com	www.example.com，.www.example.com，.example.com	example.com	example.com
g.com.cn	g.com.cn	g.com.cn，.g.com.cn	.com.cn
www.com.cn	www.com.cn	www.com.cn，.www.com.cn	.com.cn

Set cookie code:

Read cookie code:

Result screenshot: