Alternative to json_last_error() on php 5.2?

My json_decode() is failing, but I can't call json_last_error() because that function isn't available in PHP 5.2.6 (introduced in 5.3).

Is there another way for me to get the error message 开发者_开发百科so I can debug my php?

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Here is a JSON validator. Copy your JSON into it and it will tell you where the errors are.