Php session cannot be used after uploading

After php is uploaded, the session cannot be used to apply for a domain name on justhost. I use XAMPP locally to run php debugging. But after uploading it yesterday, I found many problems! I am a newbie. 1. Register for logon. if the password is incorrect, a prompt will be prompted, indicating that the password is incorrect, but the logon will not jump after the logon is successful, or on the logon page, without any prompt. why? 2. the home page will show that the session cannot be used after my SQ php Upload
I applied for a domain name on justhost. I used XAMPP locally to run php debugging.
But after uploading it yesterday, I found many problems! I am a newbie.
1. Register for logon. if the password is incorrect, a prompt will be prompted, indicating that the password is incorrect, but the logon will not jump after the logon is successful, or on the logon page, without any prompt. why?
2. the home page will show that my SQL syntax is incorrect, but everything is fine locally. only the home page prompts that the syntax is incorrect and other pages can call the database normally.
3. the following prompt appears on some pages: Warning: session_start () [function. session-start]: open (/home/whimwin1/public_html/testing/session_tmp/logs, O_RDWR) failed: No such file or directory (2) in .....

I checked it online and mentioned modifying php. ini. if this is the case, please tell me how to change it?
This website is very important to me. thank you!

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1. there are several possibilities. the header () function fails to send and so on. You need to describe your jump method. if debugging is convenient, add a line of output code before the jump code to see if the code block has been executed.
2. the homepage says your SQL statement is wrong? So what is the error? Paste the specific error information to see
3. the file storing the session is not found. Check whether the session. save_path in your php. ini file has a corresponding folder, and if it is writable
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After modifying php. ini, you still need to restart the server to make it take effect. if you do not have the permission to do so, forget it.
Make sure that the folder for storing the session is writable to everyone. I think you should have the permission to modify the folder.
In addition, if the problem persists, php also has a function that allows you to dynamically adjust the session storage location session_save_path ('Enter a writable and secure directory here ') and add it to the beginning of each session_start ().