Forskel på store og små tegn ?

Nå jeg opretter en bruger, kan jeg f.eks oprette en bruger der hedder Admin, og en mere bagefter der hedder admin, og ADmin, osv.?
Nogen der hved hvorfor ?
kode:
functions.php

function user_register($username, $password)
{
$query = mysql_query("SELECT username FROM user WHERE username='$username' limit 1");
while($row = mysql_fetch_assoc($query)) //Lav en while der kører alle rækker igennem
{
$usernametable = $row['username'];
}
if ($usernametable == $username)
{
     die ('dette brugernavn eksitere allerede!');
	 exit;
	 }
	 else
	 {
  // Reshow the form with an error
          $reg_error = 'Udfyld Venligst alle felterne.';
          include 'register_form.inc.php';
          
   
     // Get a salt using our function
     $salt = generate_salt();

     // Now encrypt the password using that salt
     $encrypted = md5(md5($password).$salt);

     // And lastly, store the information in the database
     $query = "insert into user (username, password, salt) values ('$username', '$encrypted', '$salt')";
     mysql_query ($query) or die ('Could not create user.');
	 }
}

(hved godt koden er rodet )
register_form.inc.php

<form action="register.php" method="post">

<b>Bruger:</b> <input type="text" size="20" maxlength="20" name="username"
<?php if (isset($_POST['username'])) { ?> value="<?php echo $_POST['username']; ?>" <?php } ?>/><br />

<b>Adgangskode:</b> <input type="password" size="20" maxlength="10" name="password" /><br />

<b>Gentag Adgangskode:</b> <input type="password" size="20" maxlength="10" name="confirmpass" /><br />

<input type="submit" name="submit" value="Register!" />
</form>

register.php

<?php

// Include init file
include 'init.php';

if (!isset($_POST['submit']))
{
     // Show the form
     include 'register_form.inc.php';
     exit;
}
else
{
     // Check if any of the fields are missing
     if (empty($_POST['username']) || empty($_POST['password']) || empty($_POST['confirmpass']))
     {
          // Reshow the form with an error
          print "Udfyld Venligst ALLE felterne tak.";
          include 'register_form.inc.php';
          exit;
     }

     // Check if the passwords match
     if ($_POST['password'] != $_POST['confirmpass'])
     {
          // Reshow the form with an error
          $reg_error = 'De to kodeord er ikke ens!';
          include 'register_form.inc.php';
          exit;
     }

     // Everything is ok, register
     user_register ($_POST['username'], $_POST['password']);

     echo 'Du er blevet oprette, <a href="login.php">Log ind</a>';

}

?>