[PHP] String literals keys encoding special chars in structures

Hej,

Idk if it's really an issue or a php config to set, feel free to reject this.
This piece of code :
```haxe
var o = {
"Succès" : true
}
trace( o );
trace( haxe.Json.stringify( o ) );
```
Gives the same result on all plateforms tested : Eval, Neko, HL, JS :
```
{Succès : true}
{"Succès":true}
```
But on PHP it gives that :
```
{ Succ958s : true }
{"Succ\u000195\u000e8s":true}
```
I try to insert that into MySQL and it fails... So it's not really a "Succès" 🙃

Start by running the provided Haxe structure and JSON examples on the PHP target, then compare the generated PHP and output with Eval, Neko, HL, and JS. Done means the accented key remains "Succès" in the PHP result and JSON output, without the encoded corruption shown in the issue.

Written by the indexing model from the issue text.