I have heard a lot of discussion and white papers from big corporations to the smaller ones about using NoSQL and if you look on PHP as the partner for a solution using NoSQL, then you got some good parts covered as you will see in this MongoDB and PHP article.
Fact is that NoSQL is a based document database driven, or by that I mean, it is great for articles, documents, and things of the type and if you look at it, it is basically a flat file type of database (a lot of fun for old timers like me). The rule is simple: no relation between tables (or no joins – inners, outters, etc). Everything is handled by foreign keys.
There is a lot to learn about this new “old” tech, but when considering developing systems using NoSQL consider long term systems and how your current or system in development would handle 1,000,000,000 pageviews a day and more than 200 servers. If your application depends on the servers and they can’t fail, well, revise and consider alternatives such as NoSQL.
As usual, when developing, have fun.