Php登录工作仅在mysqli_num_rows等于零时才有效，否则不起作用。我卡住了

Question

here's the code guys please help me if mysqli_num_rows==false Than code works but why num rows doesn't work i can't get it i tried everything but same error appears

 <?php
 //Start session
 session_start();

//Include database connection details
require_once('db.php');
    if(isset($_POST['submit'])){
        $username=$_POST['username'];
        $password=$_POST['password'];
        $query="SELECT * FROM users WHERE username='$username' and password='$password'";
        $result=mysqli_query($con,$query);
        if($row=mysqli_num_rows($result)==1){
            mysqli_fetch_array($con,$result);
            echo 'Logged in';
            header('location:profile.php');
        }
        else{
            echo 'error occured';
        }
    }
?>
<form method="POST">
 <input type="text" name="username" placeholder="username">
<input type="text" name="password" placeholder="password">
<input type="submit" name="submit">

</form>

Answer 1

<form method="POST">
 <input type="text" name="username" placeholder="username">
<input type="text" name="password" placeholder="password">
<input type="submit" name="submit">
</form>
<?php
error_reporting(E_ALL); // check all type of error
ini_set('display_errors',1); // display those errors
session_start();
require_once('db.php');
if(!empty(trim($_POST['username'])) && !empty(trim($_POST['password']))){ // check with posetd value
   $user_name = trim($_POST['username']);
   $password = md5(trim($_POST['password']));
   $query = "SELECT * FROM users where username='$username' and password = '$password'"; // don't use plain password, use password hashing mechanism
   $result = mysqli_query($con,$query); // run the query
   if(mysqli_num_rows($result)>0){ // if data comes
        // here do some data assignment into session
        header('location:profile.php'); // go to other page
   }else{
       echo "Login creadentials are not correct"; // else no user is there with the given credentials
   }
}else{
  echo "please fill the form value";
}
?>

Note:-

Read and use prepared statements to prevent your code from SQL Injection. :-http://us.php.net/manual/en/mysqli-stmt.prepare.php

Above file extension must be .php