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PHP Constant names and messages from the PHP manual.

json_decode() returns NULL

PHP does not throw on invalid JSON by default. json_decode simply returns NULL, which is also a legitimate result for the input "null", so you cannot tell failure from success by the return value alone. The first job is to make PHP tell you what went wrong.

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Input

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What actually causes it

Ranked by how often each one turns out to be the answer.

  1. 01 You have no error message yet

    Call json_last_error_msg() immediately after decoding, or pass the JSON_THROW_ON_ERROR flag so a JsonException is raised instead. Without one of these you are debugging blind, which is why this problem takes people so long.

    Breaks

    $data = json_decode($body, true);
if ($data === null) { /* why? */ }

    Works

    $data = json_decode($body, true, 512, JSON_THROW_ON_ERROR);
// or
$data = json_decode($body, true);
if (json_last_error() !== JSON_ERROR_NONE) {
    throw new RuntimeException(json_last_error_msg());
}

  2. 02 A UTF-8 byte order mark at the start of the file

    A classic in PHP because file_get_contents returns the BOM and json_decode rejects it. Strip the first three bytes if they are EF BB BF.

  3. 03 Malformed UTF-8, giving JSON_ERROR_UTF8

    "Malformed UTF-8 characters, possibly incorrectly encoded" usually means data came out of a Latin-1 database column. Convert with mb_convert_encoding before decoding.

  4. 04 A warning or notice printed before the JSON

    Any output before your response body ends up in it. Turn display_errors off in production and log instead.

  5. 05 Nesting deeper than the default limit, giving JSON_ERROR_DEPTH

    json_decode defaults to a depth of 512. Raise the third argument if the data is genuinely that deep.

Questions

What are the JSON_ERROR constants?
JSON_ERROR_NONE (0), JSON_ERROR_DEPTH (1, maximum stack depth exceeded), JSON_ERROR_STATE_MISMATCH (2), JSON_ERROR_CTRL_CHAR (3, unexpected control character found), JSON_ERROR_SYNTAX (4, syntax error, malformed JSON), JSON_ERROR_UTF8 (5, malformed UTF-8 characters), plus JSON_ERROR_RECURSION, JSON_ERROR_INF_OR_NAN, JSON_ERROR_UNSUPPORTED_TYPE, JSON_ERROR_INVALID_PROPERTY_NAME, JSON_ERROR_UTF16 and JSON_ERROR_NON_BACKED_ENUM.
How do I tell a NULL result from a NULL failure?
json_last_error() === JSON_ERROR_NONE means the decode succeeded, so a NULL return is the genuine value null. Any other code means it failed.

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