It's all depends on the directory formation you've set up. If a stylesheet.css is in the similar directory as header.php, you'll need to include this:
Code:
echo "<link rel='stylesheet' type='text/css' href='stylesheet.css' />";
If you have every of your exterior CSS in a directory under header.php, you'll need to include this:
Code:
echo "<link rel='stylesheet' type='text/css' href='css/stylesheet.css' />";
And last if you have your CSS file in a folder over header.php, you'll need to include this:
Code:
echo "<link rel='stylesheet' type='text/css' href='../stylesheet.css' />";