为什么php数组不能转换为JSON？

I am reading data from the mysql database and i am saving them into an array which looks like this:

Array
(
    [0] => aargau
    [1] => appenzell_au
    [2] => appenzell_in
    [3] => basel-land
    [4] => basel-stadt
    [5] => bern
    [6] => freiburg
    [7] => genf
    [8] => glarus
    [9] => graub�nden
    [10] => jura
    [11] => luzern
    [12] => neuenburg
    [13] => nidwalden
    [14] => obwalden
    [15] => obwalden
    [16] => schwyz
    [17] => solothurn
    [18] => st_gallen
    [19] => tessin
    [20] => thurgau
    [21] => uri
    [22] => waadt
    [23] => wallis
    [24] => zug
    [25] => z�rich
)

THE CODE (client.class.php) THAT DOES THIS LOOKS LIKE THIS :

public function getCantons($country){
        $cantonsArray= array();
    //  $tmpArray = array();
        $sql_get_cantons = sprintf("SELECT name FROM locations WHERE country='".$country."'");
        $result = $this->CONNECTION->query($sql_get_cantons);
        while($row = mysqli_fetch_array($result)){  
            array_push($cantonsArray,$row['name']);
        }
        return $cantonsArray;
    }

I HAVE AN OTHER PHP FILE (server.php) which returns the answer to the ajax call

if($_POST['command']=='cantons'){
        $country = $_POST['country'];
        $client = new Client();
       // $results = $client->getActiveItemsOfCategory('technology');
        $results = $client->getCantons($country);
        $jsonResults = json_encode($results);
        print_r($jsonResults);
    }

IN THE AJAX CALL (which is working properly becauseI use I use the same code as in other AJAX calls) I simply do this :

 var jqxhr = $.post( "php/server.php", {country:COUNTRY,command:'cantons'},function() {

})
  .done(function(data) {
        console.log(data);
   })
  .fail(function(data) {
     alert(JSON.stringify(data));
  })
  .always(function() {

});

THE result is empty

Try adding this to your server.php

header('Content-Type: application/json');
echo $jsonResults;

Use echo instead of print_r to output your JSON string

The problem was the encoding of the letters. Apparently, I am using German alphabet that was the reason for this error.