If the first element of an array has a numeric index, PHP treats the whole array as numerically indexed array, even if the subsequent elements of the array have a string key. 

session_start();
printf('Current session data is: <pre>%s</pre>', htmlspecialchars(session_encode()));
var_dump($foo);
$foo = array (0 => true, "foo" => "stringvalue");
session_register("foo");
print('<p>');
var_dump($foo);

This code leads to:
Current session data is: <pre>&lt;wddxPacket version='1.0'&gt;&lt;header/&gt;&lt;data&gt;&lt;struct&gt;&lt;var name='foo'&gt;&lt;array length='2'&gt;&lt;boolean value='true'/&gt;&lt;string&gt;stringvalue&lt;/string&gt;&lt;/array&gt;&lt;/var&gt;&lt;/struct&gt;&lt;/data&gt;&lt;/wddxPacket&gt;</pre>array(2) {
  [0]=>
  bool(true)
  [1]=>
  string(11) "stringvalue"
}
<p>array(2) {
  [0]=>
  bool(true)
  ["foo"]=>
  string(11) "stringvalue"
}

A possible solution would be to include always the index when serializing an array, be it numeric or string.