json_decode()'s handling of invalid JSON is very flaky, and it is very hard to reliably determine if the decoding succeeded or not. Observe the following examples, none of which contain valid JSON:

The following each returns NULL, as you might expect:

var_dump(json_decode('['));             // unmatched bracket
var_dump(json_decode('{'));             // unmatched brace
var_dump(json_decode('{}}'));           // unmatched brace
var_dump(json_decode('{error error}')); // invalid object key/value
notation
var_dump(json_decode('["\"]'));         // unclosed string
var_dump(json_decode('[" \x "]'));      // invalid escape code

Yet the following each returns the literal string you passed to it:

var_dump(json_decode(' [')); // unmatched bracket
var_dump(json_decode(' {')); // unmatched brace
var_dump(json_decode(' {}}')); // unmatched brace
var_dump(json_decode(' {error error}')); // invalid object key/value notation
var_dump(json_decode('"\"')); // unclosed string
var_dump(json_decode('" \x "')); // invalid escape code

(this is on PHP 5.2.6)

Reported as a bug, but oddly enough, it was closed as not a bug:
http://bugs.php.net/bug.php?id=45989
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