PHP Warning: Unknown: failed to open stream

I edited the apache httpd.conf file recently for the mod_rewrite to work. I don’t know if this problem originated from that or not, but i’m getting this problem from that day.
This is what I see on the frontend when I run the software –

Server error The website encountered an error while retrieving http://localhost/prestashop/. It may be down for maintenance or configured incorrectly.

There is no specific file that triggers this. But I saw that usually small programs that consists of 4-5 php files, runs fine, but softwares with a lot of files(like PrestaShop) don’t.
I checked the logs and this is what I found.

[Wed Mar 16 19:33:39 2011] [error] [client ::1] PHP Warning:  Unknown: failed to open stream: Permission denied in Unknown on line 0
[Wed Mar 16 19:33:39 2011] [error] [client ::1] PHP Fatal error:  Unknown: Failed opening required '/var/www/html/yomig/index.php' (include_path='.:/usr/share/pear:/usr/share/php') in Unknown on line 0
[Wed Mar 16 19:33:39 2011] [error] [client ::1] File does not exist: /var/www/html/favicon.ico
[Wed Mar 16 19:33:41 2011] [error] [client ::1] PHP Warning:  Unknown: failed to open stream: Permission denied in Unknown on line 0
[Wed Mar 16 19:33:41 2011] [error] [client ::1] PHP Fatal error:  Unknown: Failed opening required '/var/www/html/yomig/index.php' (include_path='.:/usr/share/pear:/usr/share/php') in Unknown on line 0

Answer

Experienced the same error, for me it was caused because on my Mac I have changed the DocumentRoot to my users Sites directory.

To fix it, I ran the recursive command to ensure that the Apache service has read permissions.

sudo chmod -R 755 ~/Sites