$username = $_POST['username'];
$password = $_POST['password'];
$result = mysql_query("SELECT * FROM Users WHERE UserName LIKE $username");

while($row = mysql_fetch_array($result))
{
    echo $row['FirstName'];
}

$sql = "SELECT ...";
$res = mysql_query($sql) or trigger_error(mysql_error()." in ". $sql);

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);

mysql_connect($host, $username, $password) or die("Ошибка подключения");
mysql_select_db($db_name) or die("Ошибка выбора БД");

$sql = "SELECT * FROM table_name";
$result = mysql_query($sql);

if ($result === false) {
    echo mysql_error();
}

$username = $_POST['username'];
$password = $_POST['password'];

if ($result = mysql_query("SELECT * FROM Users WHERE UserName LIKE $username") and mysql_num_rows($result)){
   while($row = mysql_fetch_assoc($result))
   {
      echo $row['FirstName'];
   }
}else{
   if (!mysql_num_rows($result)){
      echo "empty result";
   }else{
      echo mysql_error();
   }
}