Description:
------------
In a multi-dimensional assoc array with a depth of x, an unset key at depth x+1 evaluates unexpectedly to a string when using var_dump() - the 1st char of the array value. 

This means that isset() returns TRUE when testing for the unset key, which makes it hard to test for non-existence.

An unset key with a depth of x+2 works as expected. 

This behaviour has been replicated in 4.3.8 and the 5 previous versions, all on Linux.

Reproduce code:
---------------
$foo['one']['two'] = 'test-value' ;

if( isset( $foo['one']['two']['three'] ) )
{
    print( "Unexpected isset(): evaluated to TRUE<br>" ) ;
}
else
{
    print( "Expected isset(): evaluated to FALSE<br>" ) ;
}

print( 'var_dump() output (expecting NULL):<br>' ) ;
var_dump( $foo['one']['two']['three'] ) ;

Expected result:
----------------
As I am testing for an unset key, I expected isset() to return FALSE, and var_dump() to return NULL.

Actual result:
--------------
In fact, isset() returns TRUE.

var_dump() gives a clue as to what is going on: it returns the first character of the key's value string.

For unset keys of a depth of x + 2, isset() returns FALSE and var_dump() returns NULL, as expected.