I think your first, main example is needlessly confusing, very confusing to newbies:
$array = array(
"foo" => "bar",
"bar" => "foo",
);
It should be removed.
For newbies:
An array index can be any string value, even a value that is also a value in the array.
The value of array["foo"] is "bar".
The value of array["bar"] is "foo"
The following expressions are both true:
$array["foo"] == "bar"
$array["bar"] == "foo"