php: fixed StringTools.hex for negative values

git-svn-id: http://haxe.googlecode.com/svn/trunk@6161 f16182fa-f095-11de-8f43-4547254af6c6
php: further fix for StringTools.hex

git-svn-id: http://haxe.googlecode.com/svn/trunk@6162 f16182fa-f095-11de-8f43-4547254af6c6