Error message, wont let me log in.

Hi! I’m getting an error message at the top of my site that says:

[10-Jul-2008 20:19:28] PHP Fatal error: Call to undefined function: get_option() in /home/ganig0/public_html/wp/wp-content/plugins/wp-shopping-cart/gold_cart_files/merchants/paypal_payflowpro.php on line 10

I dont know what it is, or how to fix it. When I try to login to post or do anything, more error messages come up:

Warning: Cannot modify header information – headers already sent by (output started at /home/ganig0/public_html/wp/wp-content/plugins/wp-shopping-cart/gold_cart_files/merchants/error_log:2) in /home/ganig0/public_html/wp/wp-login.php on line 202

Warning: Cannot modify header information – headers already sent by (output started at /home/ganig0/public_html/wp/wp-content/plugins/wp-shopping-cart/gold_cart_files/merchants/error_log:2) in /home/ganig0/public_html/wp/wp-login.php on line 214

Warning: Cannot modify header information – headers already sent by (output started at /home/ganig0/public_html/wp/wp-content/plugins/wp-shopping-cart/gold_cart_files/merchants/error_log:2) in /home/ganig0/public_html/wp/wp-includes/pluggable.php on line 557

Warning: Cannot modify header information – headers already sent by (output started at /home/ganig0/public_html/wp/wp-content/plugins/wp-shopping-cart/gold_cart_files/merchants/error_log:2) in /home/ganig0/public_html/wp/wp-includes/pluggable.php on line 694

I read that the “Cannot Modify Header Information” errors were caused by white spaces at the end of the files. I checked in those files and there aren’t any white spaces at the end. I have no idea about the first error message which I suspect is what’s causing the other ones. Does anybody have any ideas on how to do this? My website is http://www.ganiggle.com