Fatal error when altering the user login form
Looks like the problem is your class definition.

['attributes']['class'] should be an array but you are setting it as a string.

Your line:

$markup .= ' ' . l(t('Sign up'), 'user/register', array('attributes' =&gt; array('title' =&gt; t('Create a new user account.'), 'class' =&gt; 'register-link')));


Should be:

$markup .= ' ' . l(t('Sign up'), 'user/register', array('attributes' =&gt; array('title' =&gt; t('Create a new user account.'), 'class' =&gt; array('register-link'))));