1.2 Required skills

PHP OOP

First, you need some programming skills :). We recommend the courses from teamtreehouse.com:
https://teamtreehouse.com/tracks/php-development

These courses are not free, but teamtreehouse.com offers a 14-day trial.

Symfony basics

To start out, you need to learn Symfony. You can find good tutorials at:
https://knpuniversity.com/

KNP university is a paid service, but free tutorials are available all over the web. We recommend searching for them as they often change or are renewed. 

For further reference, we also recommend to read the Symfony Cookbook:
http://symfony.com/doc/current/cookbook/index.html

MongoDB ODM

As Integrated uses MongoDB heavily we recommend having some knowledge about it. 

We recommend the Doctrine MongoDB ODM documentation:
http://doctrine-mongodb-odm.readthedocs.org/en/latest/

Working with Composer

Integrated uses Composer for package management. You need to know some of the basics of Composer. We recommend these courses from teamtreehouse.com:
http://teamtreehouse.com/library/php-standards-and-best-practices

These courses are not free, but teamtreehouse.com offers a 14-day trial.