You can't assign a value to a reference expression which is exactly what you try to do:

&$b = 1;

The same applies for the array operator.

Moving it over to a documentation problem so the manual outlines this better.

I understand the issue that assigning a value to a reference does not work:
  &$b = 1;
but why doesn't this work:
  &($b = 1);
which assigns the value 1 to a variable but could still maintain the reference for the rest of the expression where $b should not be automatically dereferenced.
So we could do:
  $a = &($b[] = new Object());

Work-around:
  $b[] = $a = new Object();
does not work appropriately because $b[] will receive a copy of $a but not necessarily the same object, so if we then perform:
  $a->field = 1;
then the element in $b[] will not have the added field.
Solution:
  $a =& new Object();
  $b[] =& $a;
The first line assigns to $a the same reference as the new object instead of copying it.
Then the second line adds the same reference in the array...
My view on the unary & operator is that it is not like the & operator in C or C++, but instead it's a modifier that instructs to the following expression that it must not copy the referenced object. That's why I prefer detaching it from the variable name.
But I still can't understand why the syntax: &(expression) is invalid: it should simply unflag the implicit dereference of the right-expression, so that the left-expression is not a a value but can still be a reference.

This would mean that the following would be valid:
   $a = &1;
meaning that we set $a to the same reference as the reference to the constant 1, instead of copying that constant into $a... Of course here the result would be basically the same because such a constant is not an object by itself. But at least it would have the effect of allowing autoboxing of primitive types as if they were objects, so it would unify the datatype model...

You can make references only to variables, not to expressions. It's written in the manual page language.references.whatdo: "PHP references allow you to make two variables to refer to the same content."