Is there a difference between two these functions?

Nope :)

I'm not sure but I think there is. In the first example you set the variable $greet to be the specified function. In the second example you simply create a function greet().

In the case of the first example where you set it as a variable, you can later (or earlier in your code) change that variable to be something completely different than the function. Such as for example, another function or a new integer, etc.

In the second example there is no variable, only a set function called greet() which can not be changed unless you change the code of the function itself.

That is one difference...

Why

$greet = function() {

};

Starts off with a variable, then the function ends with the a semicolon. Is there a different way to call

$greet = function() {
    echo '?';
};