PHP - limitar stock

<?php

header("Access-Control-Allow-Origin: *");

header("Content-Type: text/html;charset=utf-8");

$servidor = "localhost";

	$usuario = "user";

	$contraseña = "contraseña";

	$dbname = "coches";

    $conn = mysqli_connect($servidor, $usuario, $contraseña, $dbname);

if(isset($_POST['consulta']))

{

$q=mysqli_query($conn, "SELECT * FROM `coches` WHERE `username`='$username' and `status`='true'");

	while ($f=mysqli_fetch_object($q)) {

		$consulta[]=$f;

if ($consulta<=10){

echo "ok";

}

}

echo "Error";

}

?>

$q=mysqli_query($conn, "SELECT * FROM `coches` WHERE `username`='$username' and `status`='true'");

$fila = mysqli_num_rows($q);

if ($fila<=10){

echo "true";

}

else{

  echo "false";

}

}

SELECT COUNT(id) FROM `coches` WHERE `username`='$username' and `status`='true'

if(isset($_POST['consulta']))

{

$q = mysqli_query($conn, "SELECT * FROM coches WHERE `username`='$username' and `status`='$status'");

$consulta = mysqli_num_rows($q);

if ($consulta<=9 && ($consulta['max_capacidad']==$consulta['capacidad']))

{

echo "true";

}

else{

  echo "false";

}

}