The PHPUnit method returnValueMap() made me think a bit about how it works.
The following definition, taken from the official PHPUnit documentation, is not too verbose and I had to do few searches and few tests on my own before coming to an acceptable conclusion.
“Sometimes a stubbed method should return different values depending on a predefined list of arguments. You can use returnValueMap() to create a map that associates arguments with corresponding return values. See Example 10.6 for an example.”
(source PHPUnit Manual Test Doubles Stubs Example 10.6 )