I spent a couple of hours on a little annoying problem.
I did not find the solution here, so i add the information to this "database"...
When you try to get data (like a png, or high score ...) from a PHP server you may have some problems, like the midlet working on the emulator and not on the device, or depending on the device ...
In fact, your problem could come from the Wap Gateway, which will make some conversion on your data. My midlet worked fine one month ago, and today it works only in emulator, so obviously the bug came from the gateway of my telco (Orange France). By the way, if someone know what they did ... ?
Anyway, if it happens to you, this should be the first line of any PHP file acting as a server :
Now the gateway knows how to handle the data, and it solved my problem.
Hope it will help someone !