I don't see what this has to do with Android. Apparently the JSON that is being posted can't be handled by that PHP page. If this was my problem I'd create a Java desktop client application that gets this right before integrating it into an Android app; it's much easier to debug that way.
Not quite - it's because the PHP page is getting something other than what it can handle. Either the PHP code is incorrect, or the JSON is different than what the PHP code expects. We can't really speculate which one of these it is, but *you* can, since both the JSON and the PHP are under your control, and amenable to being changed by you.
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Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop