Login Script and Fixing Basic PHP Errors

Recently we started a fun website and used basic PHP to code it. However, it has grown more than expected (we expected it to stay community based) and there are some security problems with our login and PHP coding for the site. For an experienced programmer, I'm sure this job will be pretty easy, as it involves taking existing codes and databases that are already set up and manipulating them. Payment will depend on how many errors you find and fix.