$conn = new mysqli($servername, $username, $password, $dbname);
$result = $conn->query("SELECT firstname, middlename, lastname FROM nametable");

$outp = "[";
while($rs = $result->fetch_array(MYSQLI_ASSOC)) {
    if ($outp != "[") {$outp .= ",";}
    $outp .= '{"firstname":"'  . $rs["firstname"] . '",';
    $outp .= '"middlename":"'   . $rs["middlename"]        . '",';
    $outp .= '"lastname":"'   . $rs["lastname"]        . '"}';
}
$outp .="]";
$conn->close();
echo($outp);

[
   {"firstname":"John", "middlename": "Timothy", "lastname":"Doe"} ,
   {"firstname":"Mary", "middlename":"", "lastname":"Jane"}
]

[
   {"firstname":"John", "middlename":"Timothy", "lastname":"Doe"} ,
   {"firstname":"Mary", "middlename": null, "lastname":"Jane"}
]