Login cookies blocked after customizing hashing method

This line (at least) is being output before the cookie can be set:

echo '<h2>'.$hash.'</h2>';

Sending output to the screen (even empty lines) will prevent cookies being set.

From the PHP docs:

Like other headers, cookies must be sent before any output from your
script (this is a protocol restriction). This requires that you place
calls to this function prior to any output, including <html> and
<head> tags as well as any whitespace.

error code: 523