Confusing PDO-only problem : Can't connect through socket/Access denied/Can't connect to server (shared host)

Question

So the problem changed from what it was, i\'ll leave the original question below to prevent bad reviews on answers like I had after someone editing his question I answered :

Answer 1

I found the reason for the strange behaviour. If bind-address is different to 127.0.0.1 or 0.0.0.0 (all addresses) PDO can't connect to 127.0.0.1.