If detected PHP version is less than 5.3:
This notice is meant to teach users about speed+memory advantages of upgrading + maintaining users within the piwik requirements by warning about future requirement that they can prepare at least for 3-4 months.
In f94aef3d89bdde4cbe04c24a5c10c6f63c8c6fb7: Fixes #3877, add warning message that is displayed if a user's PHP version is less than 5.3.