Error displaying the error page: Application Instantiation Error (Windows 7 + Wamp server)

When startup joomla on a new php version 5.4.15 "Error displaying the error page: Application Instantiation Error" The Problem is the mysql extension is not running or not loaded by Php, you can check that by display the loaded extensions by usin this command :

$loaded_extensions = get_loaded_extensions();
echo '<pre>';
print_r($loaded_extensions);
echo '</pre>';
exit;

you will see n the resulted list only the default extensions That means the Apache server dont know where to fnd those extensions, I mean the extension_dir is not clearly configured in apache configuration file . to solve this issue go to php version folder, and find the file phpForApache.ini (on Wamp server) then write the right address to the extensions directory. like this :

extension_dir = "c:/wamp/bin/php/php5.4.15/ext/"

check again that this line is uncommented (the ; is removed). restart your server again and it will load the extensions from the right place. 

Note: When You change anything in phpForApache.ini for the php version  it must be not loaded, that means the php version thats hold this file must be not running.

Note: If you are not on Wamp Server you have to check you phpinfo() page to see where is your php.ini reside.