Fix PHP requirement in installation documentation

Changed minimal PHP requirement from 5.3.8 to 5.3.3, as it's the version required in composer for symfony/symfony. If 5.3.8 (or higher) is the recommended one, I'll suggest to add a comment about it.
Be more explicit that PHP 5.3.8 or higher is the recommendation
Keep the minimum viable info about PHP in the Book's installation chapter
Add a warning about PHP < 5.3.8 limitations in the requirements article
Fix linking to the requirements section.