Question

Although I get it that

$a = new b()

would be initializing an object for the class b, but what would

$a = new $b()

mean because I came across some code that happens to work otherwise!

Answer 1

It’s a reflexive reference to the class with a name that matches the value of $b.

Example:

$foo = "Bar";

class Bar
{
   ...code...
}

$baz = new $foo();

//$baz is a new Bar

Update just to support: you can call functions this way too:

function test(){
    echo 123;
}
$a = "test";
$a(); //123 printed