what is ideal phase to start testing and why?

Answer 1

There are different kinds of testing for each stage of the project. Start incremental testing during development. Then run full tests after that.

Answer 2

Test planning should begin in the Design phase.

This allows for development of robust test suites in sufficient time for deployment in the Development phase, the actual start of testing.

There is nothing to actually test before something is built, but it is important to begin testing as soon as something is built.

Once in the Development phase, the first round of testing, Unit testing, ensures the integrity of essential "building blocks". This is important, as the further the project progresses, the more complicated and expensive resolution of discrepancies becomes.

Once the Unit test is successful, the building blocks can be assembled for test round 2, String Test. This is important to ensure the individual building blocks fit together well.

System test, regression test, etc... success are all contingent upon satisfactory performance of those elements first produced early in the Development phase, as well as each element successively integrated thereafter.