<?php
   $con=mysqli_connect("localhost","user","senha","banco");
   $parametro = $_GET["parametro"];
   if (mysqli_connect_errno()){ echo "Failed to connect to MySQL: " .mysqli_connect_error(); }
   $sql = "SELECT * FROM " . $parametro;

   if ($result = mysqli_query($con, $sql))
   {
        $resultArray = array();
        $tempArray = array();

    while($row = $result->fetch_object())
    {
        $tempArray = $row;
        array_push($resultArray, $tempArray);
    }
    echo json_encode($resultArray);
}
mysqli_close($con);
?>

$str = str_replace('u','u',$decoded);
$strJSON = preg_replace('/u([da-fA-F]{4})/', '&#x1;', $str);

echo json_encode($json, JSON_UNESCAPED_UNICODE);

echo json_encode($json);

<?php
       $con=mysqli_connect("localhost","user","senha","banco");
       $parametro = $_GET["parametro"];
       if (mysqli_connect_errno()){ echo "Failed to connect to MySQL: " .mysqli_connect_error(); }
       $sql = "SELECT * FROM " . $parametro;

       if ($result = mysqli_query($con, $sql))
       {

        while($row = $result->fetch_object())
        {
            foreach($row as $key => $col){
               $col_array[$key] = utf8_encode($col);
            }
            $row_array[] =  $col_array;

        }
        echo json_encode($row_array);
    }
    mysqli_close($con);
    ?>

$resultArray = array_map('utf8_encode', $resultArray);

$result = converteArrayParaUtf8($result); //chama a função
return json_encode($result); //retorna

function converteArrayParaUtf8($result){
    array_walk_recursive($result, function(&$item,$key){
         if (!mb_detect_encoding($item, 'utf-8', true)) {
                $item = utf8_encode($item);
            }
    });
    return $result;
}