include 'connectMySql.php';
if(! $conn )
  die('Could not connect: ' . mysql_error());
$sql = 'SELECT * FROM users ';

$result = mysql_query( $sql);
if(! $result)
  die('Could not get data: ' . mysql_error());

while($row = mysql_fetch_assoc($result, MYSQL_ASSOC))
    echo "ID       : {$row['id']}  <br> ".
         "USERNAME : {$row['username']} <br> ".
         "EMAIL    : {$row['email']} <br> ".
echo "Fetched data successfully\n";

i want to select the row where username="$username"

What do you mean by "select"? Do you mean click on some row in a table displayed on your page? Or do you mean you only want to select one row from the DB where username="$username"

$sql = 'SELECT * FROM users WHERE username="' . $username . '" LIMIT 1');

Use LIMIT1 in your SQL statment

i have this problem:
Undefined index: username in C:\xampp\htdocs\PhpProject2\profile.php
and when i open my project again :
help me!

You need to define the variable $username, or if it's where you do USERNAME : {$row['username']} <br /> — you might have 'username' miss-spelled in your database.