Databas problem! eller kanske PHP..

<?php

include ('mysql.php');

if (isset ($_POST['submit'])) {
	$username = mysql_escape_string ($_POST ['username']) ;
	$password = mysql_escape_string (sha1($_POST ['password'])) ;
	
	if ( !empty ($username) && !empty ($password)) {
		$sql = mysql_query (" INSERT INTO users (user_id,username,password)
						VALUES ('0',' ".$username." ',' ".$password." ')
					   ");
										
		echo 'you are now registered';	
									
	} else {
		echo 'you must enter a pasword and a username';
	}
} else {
	echo '<form action="register.php" method="post">
			username: <input type="text" name="username" /><br />
			password: <input type="password" name="password" /><br />
			<input type="submit" name="submit" value="register"><br />
			</form>' ;
}

?>

<?php

session_start();

include ('mysql.php');

if (isset ($_POST['submit']))  {
	$username = mysql_escape_string ( $_POST ['username']) ;
	$password = mysql_escape_string ( sha1($_POST ['password'])) ;
	
	if (!empty ($username) && !empty ($password)) {
		$sql = mysql_query ("SELECT * FROM users
		WHERE username=' ".$username." '
		AND password=' ".$password." '
		LIMIT 1");
		
			if (mysql_num_rows ($sql) > 0) {
			$_SESSION['loggedin'] = true;
			$_SESSION['username'] = $username; 
			
			echo 'You are now log in';
	} else {
		echo 'your username and/or password is incorrect!';
	}
} else {
	echo 'You must enter a username anda password';
}

	

} else {
			echo  '<form name="login" action="login.php" method="post">
					<label><h4>Username:</h4></label>
					<input type="text" name="username" />
					
					<label><h4>Password:</h4></label>
					<input type="password" name="password" />
					
					<input class="submit" type="submit" value="Go" name="submit" />
			</form> login failed';
		}
?>