Php class and object problems

Php class and object problems PHP object class

Why not?
Public $ mp = new Model ("producttype ");
In this way, I can do this in C #. it can be the object but the attribute. The package in php is wrong. why.

Reply to discussion (solution)

The member property value must be a definite value,

That is to say, the value type should be: basic data type, which can be an array, but not an object.

C # is compiled and executed, so it can be used in that way
Php is interpreted and executed. it is impossible to predict what the new Model ("producttype") is in the syntax analysis phase.

In fact, C # does not know what the new Model ("producttype") is during syntax analysis.
But he still has a compilation process that can handle it.
Php does not use this association process. Instead, it starts to execute the code after the syntax analysis, while executing and interpreting the code.

Oh, thank you. I understand