PHP - Web/list Question...

You could paginate the list, in a simple form, but include at the top the values already selected on other/previous pages. You also might consider letting the user type a few letters of what they want, and show them the matching results to be added. This presumes most users have a familiarity with the possible selections. Another option might be to put in a collapsing tree-like structure a la GUI file-systems like Mac OS. So all the values would be on the page, but they'd show/hide the hierarchical groupings of interest. But this only makes sense if the data can be intuitively laid out as a hierarchy. And it presumes Javascript-enabled viewers, which may or may not fit long-term plans involving cell phones and/or disabled users and/or whatever. [The preceding sentence is an attempt to head off the endless JS thread that often seems to follow a JS solution]