PHP json_decode() returns null

Getting the real error

There are two ways to find out what went wrong. The first works on any PHP version; the second, available since PHP 7.3, makes json_decode() throw an exception instead:

$data = json_decode($json, true);
if (json_last_error() !== JSON_ERROR_NONE) {
    echo json_last_error_msg();   // e.g. "Syntax error"
}

// PHP 7.3+: throw instead of returning null
try {
    $data = json_decode($json, true, 512, JSON_THROW_ON_ERROR);
} catch (JsonException $e) {
    echo $e->getMessage();
}

If you can, use JSON_THROW_ON_ERROR. Besides giving you the message, it means a failed decode can never be mistaken for JSON that really is null.

What each message means

Syntax error is the one you’ll see most, and unfortunately it doesn’t say where the problem is. Pasting the JSON into the fixer above, or the validator, will point to the exact line.

Syntax error on JSON that looks fine

If the JSON looks fine and even passes a validator, but PHP still says Syntax error, the file probably starts with an invisible byte order mark. Some editors on Windows add one when saving as UTF-8. Strip it before decoding:

$json = preg_replace('/^\xEF\xBB\xBF/', '', $json);

Malformed UTF-8 characters

json_decode() only accepts UTF-8. This error means the text is in another encoding, often Windows-1252 or Latin-1 from an older database or file. You’ll usually notice it on accented characters. Convert it to UTF-8 first. Or, if losing the odd character is acceptable, have PHP replace the bad bytes:

// Convert from the real encoding
$json = mb_convert_encoding($json, 'UTF-8', 'Windows-1252');

// Or replace invalid bytes with U+FFFD (PHP 7.2+)
$data = json_decode($json, true, 512, JSON_INVALID_UTF8_SUBSTITUTE);

If the text comes from MySQL, it’s better to fix it at the source: set the connection charset to utf8mb4, and the data will arrive as UTF-8 in the first place.

The same error elsewhere