"" is a tricky case because in this case $a is converted to array. So strictly speaking [] is not applied to a string, it applies to an array, to which empty string is auto-converted.

stas, I respectfully disagree.  Firstly,
https://github.com/php/php-langspec/blob/master/spec/08-conversions.md#converting-to-array-type
says

    If the source type is bool, int, float, or string, the result value
    is an array of one element whose type and value is that of the
    source.
    
but that's not happening here as

    $a =  ""; $a[] = "y";
    
gives

    array(1) { [0]=> string(1) "y" }

whereas the conversion of the empty string to an array with an empty 
string as its first element should have given

    array(2) { [0]=> string(0) "", [0]=> string(1) "y" }

So the empty string is becomming an empty array.  I don't see this
possibility stated anywhere. 

"Subscript Operator" doesn't say auto-conversion applies.  If that's 
really what's happening here, then shouldn't it?

The values of false, null and "" are all converted to an empty array and thus do not follow the traditional casting rules.

This is also evident from the source code: http://lxr.php.net/xref/PHP_5_6/Zend/zend_execute.c#1160

This should probably be mentioned in the spec.