json_encode is returning NULL?

Solution 1:

I bet you are retrieving data in non-utf8 encoding: try to put mysql_query('SET CHARACTER SET utf8') before your SELECT query.

Solution 2:

If you have at least PHP 5.5, you can use json_last_error_msg(), which will return a string describing the problem.

If you don't have 5.5, but are on/above 5.3, you can use json_last_error() to see what the problem is.

It will return an integer, that you can use to identify the problem in the function's documentation. Currently (2012.01.19), the identifiers are:

0 = JSON_ERROR_NONE
1 = JSON_ERROR_DEPTH
2 = JSON_ERROR_STATE_MISMATCH
3 = JSON_ERROR_CTRL_CHAR
4 = JSON_ERROR_SYNTAX
5 = JSON_ERROR_UTF8

These can change in future versions, so it's better to consult the manual.

If you are below 5.3, you are out of luck, there is no way to ask what the error was.