I assume latent type means simply the implicit typing. i.e. you declare a variable with the value, and the type is inferred through the value of the variable rather than the explicit type declared.
As usual, the abstract really isn't enough to draw big conclusions from. The concept of a latent type applies to arbitrary expressions, not just variables. All the values flowing through a PHP program implicitly share the same union type; they can be floats, strings, arrays, etc. The latent types are the narrower types that can actually flow through the program in practice.
To be clear, it is more general than just things like:
$a = 0; // $a is an int!
g(foo() . bar());