Both purchasing power parity (PPP) and the big mac index are based on the concept of valuing a currency based on what goods and services it can buy in it's local market. However the big mac index is based on just one product, i.e. the big mac burger (in fact big mac index is a subset of ppp). Whereas purchasing power parity factors are based on about 3245 products and services.
The big mac index essentially compares the prices of the big mac burger in two different countries in their respective currencies. There is a fundamental problem with that, the big mac is not even available in many countries, thus you cannot calculate an exchange rate for such countries. Also, most Asian countries (and many other countries) are very very matured civilizations with a highly evolved local cuisine which is available quite freely and is inexpensive. A corporate like McDonald's has tried and failed to gain any respectable market share in this market. Hence all international fast food chains such as McDonald's, KFC, Subway, etc are instead serving the upper-mid market segment in Asian countries unlike the lower to lower-mid market segment they serve in the USA and some other western countries. In Asia these fast food chains use their international brand image to pitch their food to higher earning customers at higher prices. So when you use the big mac index between say, India and the USA, you are essentially comparing an upper-mid market product with an inexpensive lower market product, hence you cannot expect a good ballpark valuation.
The second level problem with the big mac index is that in many mature civilization countries the big mac is not available in its original form, instead a loose variation of the big mac is produced. And these big mac variations are not the best selling products at McDonalds (unlike in the USA where the big mac is a fast selling product); instead locally adapted dishes that McDonalds has innovated to cater to the local palate, sell more units. Since the big mac variations don't sell well, they are priced higher in such countries. Another reason why the big mac index will not give a good ballpark valuation.
The primary reason why the big mac index has gained some kind of prominence is because it was taken up for publication by a big brand 'The Economist'. It, as a concept and a method of valuation is rudimentary and is ill advised.
In comes purchasing power parity.
PPP derives its exchange values from how much local currency would you need in a country to purchase stuff like food, housing, transport, entertainment, medical services, legal services, education, the list goes on to around 3000 consumer goods, 30 government occupations, 200 equipment goods and 15 types of construction projects; everything that goes into computation of GDP (read more here). This value is derived for (almost) the same 3245 goods and services for each country. After this is done you can now compare how much money would you need in local currency to buy stuff in this country vs that country vs another country vs yet another country. For the same reason Purchasing Power Parity is the method of choice if you wish to compare your income in your country with your target income in another country.
Approaching price comparison by PPP method is desirable because it factors in the differences in resource availability (hence price differences) in different countries. For instance seafood is cheap in coastal countries but comes at a premium in inland countries. Pineapples are cheap and of good quality in Costa Rica and Côte d'Ivoire (you will need CRC 137, 075.18 to buy the same things you do with XOF 100,000 in Côte d'Ivoire) but are relatively expensive in the USA. To sustain a living one consumes a fairly large mix (basket) of goods and services rather than just one good. Hence it becomes important to evaluate prices across the spectrum of 3245 goods and services (unlike the big mac index which is based on just one product) so that such price polarities are accounted for. So while real-estate might be expensive in one country, it might be offset by an inexpensive healthcare system.
For these reasons purchasing power parity is an excellent method to compare salaries, incomes, and expenses and living standards.
