Interpolation (double quoted string) of Associative Arrays in PHP

When interpolating PHP's string-indexed array elements (5.3.3, Win32)
the following behavior may be expected or not:

$ha = array('key1' => 'Hello to me');

print $ha['key1'];   # correct (usual way)
print $ha[key1];     # Warning, works (use of undefined constant)

print "He said {$ha['key1']}"; # correct (usual way)

print "He said {$ha[key1]}";   # Warning, works (use of undefined constant)

print "He said $ha['key1']";   # Error, unexpected T_ENCAPSED_AND_WHITESPACE
print "He said $ha[ key1 ]";   # Error, unexpected T_ENCAPSED_AND_WHITESPACE
print "He said $ha[key1]";     # !! correct (How Comes?)

Inerestingly, the last line seems to be correct PHP code. Any explanations?
Can this feature be trusted?

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Edit: The point of the posting now set in bold face in order to reduce misunderstandings.

Answer

Yes, you may trust it. All ways of interpolation a variable are covered in the documentation pretty well.

If you want to have a reason why this was done so, well, I can't help you there. But as always: PHP is old and has evolved a lot, thus introducing inconsistent syntax.