<?php
  // mysql_connect is deprecated and not secure
  // You can pass the DB name as the fourth argument of mysqli_connect
  $conn = mysqli_connect('localhost','accounting','', 'ffdbR4fsa');

  if (! isset($_SESSION)) {
    session_start();
  }
?>

<form action="" method="post">
  <table width="50%" border="0">
    <tr>
      <td><h3>Admin Log In</h3></td>
    </tr>
    <tr>
      <td><label>Username</label><input type="text" name="username" placeholder="Username"></td>
    </tr>
    <tr>
      // Password type was missing
      <td><label>Password</label><input type="password" name="password" placeholder="Password"></td>
    </tr>
  </table>
  <input type="submit" value="Log In" name="s">
</form>
<?php

if (isset($_POST['s'])) {
  // the post field must match the input field name, changed to username to match
  $user = $_POST['username'];
  $pass = $_POST['password'];

  // again mysq_query is deprecated
  $result = mysqli_query($conn, "select * from admin_list where user_name ='$user' and password = '$pass'");

  // agai mysq_num_rows is deprecated
  if (mysqli_num_rows($result) === 1) {
    $_SESSION['is_admin'] = True;
    $_SESSION['user'] = $user;
    echo "Logged in as $user";
  } else {
    $_SESSION['is_admin'] = False;
    $_SESSION['user'] = NULL;
    echo 'Invalid login or password';
  }
}

// Remove the closing tags, just a good practice so another file can continue
// Some ; were missing too