On many univs websites I have found statement "apply as early as possible"
If I'll apply in February will I have bigger chances of being accepted than if I'll apply in for eg. March?
How candidates are selected?
If I'll apply early, and they decide that I'm suitable candidate, do they accept me, or wait till let's say June, and then select the best candidate?

Funding.Similar question.Why applications for research awards and masters awards should be submitted as soon as possible?

Why PHD in UK is only 3 years long, and in US 5-7?

Yes, many departments and supervisors will appoint the first good student they find, so applying earlier means you will have more choice (although some posts are not advertised until later in the year).

Funding can be hard to come by - particularly for international students, who may have to apply to a number of different sources for funds. Each of these sources will have different application deadlines, and the sooner you start applying the more chances you have.

PhDs take different amounts of time in different countries. The UK has the shortest time. The explanations are both historical and sometimes logical, but mostly tedious and would take up too much space.