Generated PHP for Thrift sets are incompatible with PHP

In the `TBase` class that all Thrift generated code inherits from, there is a `_writeList` method that takes a boolean of whether or not the "list" in question is a set. The assumption here is that since PHP has no built-in Set data type, we can duplicate the uniqueness constraint of Sets by looking at the keys of a PHP associative array. Here's the associated code in `_writeList`:

```
foreach ($var as $key => $val) {
$elem = $set ? $key : $val;
if (isset($ewrite))

{ $xfer += $output->$ewrite($elem); }

else {
switch ($etype) {
case TType::STRUCT:
$xfer += $elem->write($output);
break;
```

I want to point out again that if we're using a `TType::SET`, the `$set` variable will be `true`, and `$elem` will be the `$key`, not the `$value`. Here's the problem, from PHP array documentation (http://php.net/manual/en/language.types.array.php):

"The key can either be an integer or a string. The value can be of any type...Arrays and objects can not be used as keys. Doing so will result in a warning: Illegal offset type."

In other words, it is impossible to implement a Thrift set of anything other than strings or integers given the current implementation of PHP generated code.