First, A multiple select list will only send all of the selected options in PHP if it properly named as an array (the name needs to end in "[]"). If it is not named to create an array, only the3 last selection made will be passed. the select tag should look like:
php:
<select name="username[]" size="12" multiple="multiple">
using the implode function on an array will not create a separate record for each selection, it will convert the array to a comma separated list.
If you wish to save a unique record for each selection, you will need to hand code the insert, there is not a tool that will do this for you. You will need to code a loop through each index in the array, and execaute an SQL insert statement for each selected user name.
It will be easier to store the comma separated list in a single record.
