Well after 2 hours of digging (cf Edits)

I found out following :

• In my case it's a encoding problem
• mb_detect_encoding returns probably a faulty response, some strings were probably not UTF-8
• using utf8_encode() on those string solved my problem, but see note below

Here is a recursive function that can force convert to UTF-8 all the strings contained in an array:

function utf8ize($d) {
    if (is_array($d)) {
        foreach ($d as $k => $v) {
            $d[$k] = utf8ize($v);
        }
    } else if (is_string ($d)) {
        return utf8_encode($d);
    }
    return $d;
}

Use it simply like this:

echo json_encode(utf8ize($data));

Note: utf8_encode() encodes ISO-8859-1 string to UTF-8 as per the docs so if you are unsure of the input encoding iconv() or mb_convert_encoding() may be better options as noted in comments and other solutions.

Recommended method

json_decode ("{}") will return a stdClass per default, using the below should therefor be considered safe/portable and correct.

json_encode (new stdClass);

Your solution could work..

The documentation specifies that (object) null will result in an empty object, some might therefor say that your code is valid and that it's the method to use.

PHP: Objects - Manual

If a value of any other type is converted to an object, a new instance of the stdClass built-in class is created. If the value was NULL, the new instance will be empty.

.. but, try to keep it safe!

Though you never know when/if the above will change, so if you'd like to be 100% certain that you will always will end up with a {} in your encoded data you could use a hack such as:

$empty = json_decode ("{}");
$result = json_encode($empty); // "{}"

Even though it's tedious and ugly I do assume/hope that json_encode/json_decode is compatible with one another and always will evaluate the following to true:

$a = <something>;

$a === json_decode (json_encode ($a)); 

The array you show has all the properties as private. this mean that this value are not available outside their class's scope.

you can look at this SO for some suggestion

Using json_encode on objects in PHP (regardless of scope)

use the JSON_FORCE_OBJECT option of json_encode:

json_encode($status, JSON_FORCE_OBJECT);

Documentation

JSON_FORCE_OBJECT (integer) Outputs an object rather than an array when a non-associative array is used. Especially useful when the recipient of the output is expecting an object and the array is empty. Available since PHP 5.3.0.

Or, if you want to preserve your "other" arrays inside your object, don't use the previous answer, just use this:

$status = array(
                "message"=>"error",
                "club_id"=>$_club_id,
                "status"=>"1",
                "membership_info"=> new stdClass()
                );

You really should include the actual JSON in your question; without it we are working blind, but I can guess what the problem is. You are not constructing your JSON properly when there are multiple records. When you have a single recipient, your JSON might look like this:

{"readyState":4,"responseText":"","status":200,"statusText":"OK"}

That is fine, and perfectly valid JSON – and there is no issue having an empty string as a value. The problem arises when you have more than one record, when it will append them immediately after the first:

{"readyState":4,"responseText":"","status":200,"statusText":"OK"}{"readyState":4,"responseText":"","status":200,"statusText":"OK"}

This is not valid JSON.

I suggest you accumulate your JSON responses inside the loop and then output them afterwards, in the correct format. Something like:

$jsonData = [];
foreach ($emails_room_f as $email_room_f){
    //set your content in $data as you are now, then:
    $jsonData[] = $data;
}
header("Content-Type: application/json; charset=utf-8", true);
echo json_encode($jsonData);

This way you will end up with data like this:

[
    {"readyState":4,"responseText":"","status":200,"statusText":"OK"},
    {"readyState":4,"responseText":"","status":200,"statusText":"OK"}
]

Which is valid JSON. It will also work if there is no data as it will return an empty array, which is also valid JSON.

Separately from this, I am extremely suspicious of you having a utf8ize function – that is a sign you're doing other UTF-8 stuff wrong, but that's a subject for another question.

It is because your class's properties are private.

An example class with only private properties ...

php > class Foo { private $bar = 42; }
php > $obj = new Foo();

do not expose values:

php > echo json_encode($obj);
{}

But an example class with public properties ...

php > class Bar { public $foo = 42; }
php > $objBar = new Bar();

do it!

php > echo json_encode($objBar);
{"foo":42}

\JsonSerializable

PHP provide an'interafce \JsonSerializable that require a method jsonSerialize. This method is automatically called by json_encode().

class JsonClass implements JsonSerialize {
    private $bar;
    public function __construct($bar) {
        $this->bar = $bar;
    }
    public function jsonSerialize() {
        return [
            'foo' => $this->bar,
        ];
    }
}

I prefer this solution because is not good to expose publicly properties

serialization and unserialization ...

If you need to serialize and unserialize php object you can ...

php > class Classe { public $pub = "bar"; }
php > $obj = new Classe();
php > $serialized = serialize($obj);
php > $original = unserialize($serialized);
php > var_dump($original);
php shell code:1:
class Classe#2 (1) {
  public $pub =>
  string(3) "bar"
}

$serialized variable contains O:6:"Classe":1:{s:3:"pub";s:3:"bar";}. As you can see is not a json, but is a format that allow you to recreate original object using unserialize function.

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