If you are getting this error, you are probably trying to use Apache, MySQL and PHP in a windows system. Just do this simple step to solve this issue.
It is quite frustrating to see that all your PHP pages suddenly doesn't seem to work. If you check your Apache log file you will see this error: Parent: child process exited with status 3221225477 -- Restarting
Don't worry just goto the PHP installation folder and look for libmysql.dll file.
Just copy this file and save it to your windows\system32 folder.
Now restart MySQL and Apache servers.
Everything should be fine now and all your PHP pages will work as normal.