rest - Posting Data from Ionic App to PHP-File on Webserver via POST wont work

right now im trying to send datat from my ionic app to a php-file which should use that data for a MySQL-statement. unfortunately the file cant access the data and since im kinda new to HTTP-Requests i dont know why. As far as i can see the request is a valid one.

Below u can see my php code.

i Hope you can help

header('Access-Control-Allow-Origin: *');
header('Access-Control-Allow-Methods: GET, POST, PATCH, PUT, DELETE, 
OPTIONS');
header('Access-Control-Allow-Headers: Origin, Content-Type, X-Auth- 
Token');

$postdata = file_get_contents("php://input");
$argument = json_decode($_POST["argument"]);


$mysqli = new mysqli(//DB connection is set here);
$query = $argument;
$dbresult = $mysqli->query($query) or trigger_error($mysqli->error . " 
            [$query]");


while ($row = $dbresult->fetch_array()) {

   $json_row = $row['num'];

}

if ($dbresult) {

   $result = $json_row;

} 

else {

    $preresult = "{'success':false}";
    $result = json_encode($preresult);

}


    echo json_encode($json_row);
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