PHP cannot connect to mysql when run inside Apache

I ran into a very strange issue when Apache/PHP/MySQL.
What happend was my php code suddenly cannot connect to mysql on another machine any more. The error message is:

mysqli_connect() [function.mysqli-connect]: (HY000/2003): Can't connect to MySQL server on '218.241.156.222' (13)

The strange thing was that if I connect mySQL from command line, even run the same code by php directly from command line, it worked just fine.

After searched around, I finally found the answer. Looks like it has something to do with SELinux settings.

To fix the issue, I only need to run:

setsebool httpd_can_network_connect_db 1

Before I ran the script, the file content of /selinux/booleans/httpd_can_network_connect_db was:
0 0

After I ran the script, content changed to:
1 1