you can write javascript that fills the hidden form element with all the values from the selection, something like below, and on the server just use explode (",", $ _ POST ["allValues"]) to get all the parameters
<script> var hiddenValues = ""; $(document).ready(function(){ $("#mySelect option").each(function(){ hiddenValues = $(this).val() + ","; })</script>
obviously the above has a jQuery dependency and your form has the identifier myForm and that your multiselect has the id mySelect :)
EDIT:
NOTE 1: this saves only parameter values, not labels from select (a similar method can be used to save these parameters). just keep that in mind
NOTE2: be careful if the values contain any commas, as this will invalidate your input (if not escaped in any way or if no other delimiter is used)
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