Suppose I am troubleshooting a client site that is not loading properly and I find find this error on the php error_log:

[Mon May 28 04:56:09 2012] [error] [client 109.182.33.xx] PHP Warning: require(): Failed opening './includes/login.php' (include_path='.:/usr/share/pear:/usr/share/php') in /var/www/public_html/index.php on line 5
[Mon May 28 04:56:09 2012] [error] [client 109.182.33.xx] PHP Fatal error: Call to undefined function login() in /var/www/public_html/index.php on line 12

1. What's going on?
2. Why the PHP fatal error?
3. How can I fix it?

It seems self explanatory, it fails to open login.php file which is supposed to be in /var/www/public_html/includes directory

Thanks for the reply. How can I fix it ?

Really?? Since it's looking for the login.php file in /var/www/public_html/includes, wouldn't it make sense to COPY THAT FILE to that location??

The first thing to check is whether you actually have a public_html directory under /var/www/. I would be surprised if you did. Generally, you will have the apache files including index.html under /var/www/html or htdocs depending upon which distribution you are using which you have not indicated. The public_html directory should be in the /home/user directory and you have to enable/configure this as it usually isn't set up by default. You need to load the userdir_module for your distribution.

Check your DocumentRoot in the apache httpd.conf file and if you don't resolve this, indicate which Linux you are using in your next post.

Of course there is a public_html directory under /var/www, PHP message reports the script where the error came from

... or the file is there, but its permissions do not allow reading it