PHP MYSQL INT 0 on mysqliquery - php

Question

im new student of php and mysql. I'm doing a voluntary little game for class. But I can not understand why when doing mysqli query it returns an int 0 when the database has 500 assigned. The game consists of a user who bets a certain amount of coins of his account, then a dice decides whether you win or not, taking 1,2,3 you lose, 4,5,6 you win. I post here the error or whatever this is.
include('conexion.php');
include('session.php');
$usuario=$_SESSION["username"];
$apuesta =$_POST["apuesta"];
$coins= "SELECT coins FROM `users` WHERE `usuario` = \"$usuario\"";
if (mysqli_query($conn, $coins)){
echo "<br>";
echo "hola";
echo "<br>";
var_dump($coins);
echo "<br>";
$coins2 = (int)$coins;
var_dump($coins2);
echo "<br>";
echo "<br>";
var_dump($coins2);
echo "<br>";
if ($apuesta>=$coins2) {
echo "No tienes suficiente saldo";
echo "<br>";
}else {
$dado = rand(1,6);
if ($dado >3) {
$apuesta = $coins2 + $apuesta;
$cambiocartera = "UPDATE `users` SET `coins` = $apuesta WHERE `users`.`usuario` = '$usuario'";
echo "<br>";
echo "<h1>Ganaste = $dado";
header( "refresh:3;url=dashboard.php");
}
elseif ($dado<4) {
$apuesta = $coins2-$apuesta;
$cambiocartera = "UPDATE `users` SET `coins` = $apuesta WHERE `users`.`usuario` = '$usuario'";
echo "<br>";
echo "<h1>Perdiste = $dado</h1>";
header( "refresh:3;url=dashboard.php" );
}
}
}else {
echo"error";
}
My browser message about the int on 0 that i dont understand why.

Answer 1

$coins is the string containing the query, it's not the column you're retrieving from the table. When you convert a string to a number, it returns 0 if there's nothing numeric at the beginning.
You need to assign the result of mysqli_query() to a variable, then call a fetch function on that.
if ($result = mysqli_query($conn, $coins)) {
$row = mysqli_fetch_assoc($result);
$num_coins = $row['coins'];
var_dump($num_coins);