[Q] Parser behavior in a string

I noticed that PHP4 accept and does not raise error (notice messages) for
following line.

$arr = array('first' => 'one');
$str = "This is first element of array: $arr[first]";
//Following line cause parse error
//$str = "This is first element of array: $arr['first']";

Question: Is this expected behavior that PHP parse w/o errors?

If this is not a expected behavior and subject to change, I'm not going to use
array w/o ' or " for assoc arrays. Because I'm going to log all error messages
for production system including notice messages.

Thank you.

--
Yasuo Ohgaki