I was having this problem because I was trying to connect to MySQL but I didn't have the required package. I figured it out because of @Amadan's comment to check the error log. In my case, I was having the error: Call to undefined function mysql_connect()
If your PHP file has any code to connect with a My-SQL db then you might need to install php5-mysql first. I was getting this error because I hadn't installed it. All my file permissions were good. In Ubuntu, you can install it by the following command:
sudo apt-get install php5-mysql
