javascript - Sina Cloud Server can only use form to request session, but ajax cannot request it??

When writing the php verification code page, I stored the php file on the Sina cloud server. I thought about using ajax request session in the javascript page to verify the verification code. I feel that this method should be feasible, but in reality During the operation, I found that the session retrieved using ajax was always empty and was not requested. One of the reasons was that the code was written incorrectly. I pondered hard and looked it up in various ways. Finally, I found that netizens said that Sina Cloud could not save the session. , but when I use form to make a request, I can get the session value. Why is this? Please solve...

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When writing the php verification code page, I stored the php file on the Sina cloud server. I thought about using ajax request session in the javascript page to verify the verification code. I feel that this method should be feasible, but in reality During the operation, I found that the session retrieved using ajax was always empty and was not requested. One of the reasons was that the code was written incorrectly. I pondered hard and looked it up in various ways. Finally, I found that netizens said that Sina Cloud could not save the session. , but when I use form to make a request, I can get the session value. Why is this? Please solve...

If you don’t use a framework, there must be session_start() at the front of the PHP file, and then $_SESSION will work in the file

<code><?php
session_start();</code>