Using a drop down box with an udetermined field amount

What I want to do:
Have a person input information about individuals into a database. One of
the fields in the database will be Unit.
On a form one of my drop down menu's will be Unit. I would like to have the
PHP page poll the Unit field in the database and send all the different Unit
results to the drop down box.

Unit:<td><INPUT SIZE=6 NAME="Unit" value="">

This would be an example of the information that would be an entered
H Co
H Co
B Co
A Co
K Co

This should be the results
H Co
B Co
A Co
K Co

Any help would be appreciated!