While installing a testing version of MediaWiki on a VM instance, the welcome page warned that an environment check had failed:

Warning: The intl PECL extension is not available to handle  
Unicode normalization, falling back to slow pure-PHP  
implementation.  
  
If you run a high-traffic site, you should read a little on  
Unicode normalization. 

I had been careful (as always) to use the following line in my nginx.conf:

use utf-8;

In this case though, php5-intl needed to be installed:

# apt-get install php5-intl

The resulting download was 22.5MB, which seemed a little large. Regardless, after downloading, the intl.ini mod was loaded and the PHP5 FastCGI Process Manager reloaded:

==> Setting up libicu48:amd64 (4.8.1.1-12+deb7u1) ...  
==> Setting up php5-intl (5.4.4-14+deb7u10) ...  
==> Creating config file /etc/php5/mods-available/intl.ini   
==>     with new version   
==> Processing triggers for php5-fpm ...  
==> [ ok ] Restarting PHP5 FastCGI Process Manager: php5-fpm.

Now the MediaWiki install environment check reported:

Using the intl PECL extension for Unicode normalization.