Is it possible to get a Ph.D. position in Europe after a one-year diploma? I had interviewed by a professor for a Ph.D. position in Australia and the application has been unsuccessful. However, the professor wrote I did well in the interview and encouraged me to apply for a one-year diploma or two years master's (As I only have a 4 years bachelor's degree). I want to know what is the main difference? I mean if I do a diploma, will I be eligible for doing a Ph.D. after that? Especially in Europe (as Europe needs a master's for getting a Ph.D.)?
You are eligible for a PhD in some European countries with just a bachelor's degree. There may exist countries - e.g., I believe Germany -* where a master's degree is universally required before a PhD. In other countries, requirements vary between institutes (and supervisors may even insist upon their own requirements). So a list of countries probably isn't useful and exceptions would no doubt exist.

* A comment clarifies that a master's degree is not a universal requirement in Germany. Another comments notes, "it might not be possible to employ the PhD student as a scientific employee (Wissenschaftlicher Mitarbeiter), which is the default for PhD students in many fields," so, for practical purposes I presume a master's is a widespread norm in Germany.