To have a multiple select list pass all values that where selected in PHP, you must name the select list with "[]" at the end of the name to create an array. For example:
<select name="result[]" size="4" multiple id="result[]">
To get the values out of the array, you need to use the php implode function:
<?php echo(implode(", ", $_POST['result'])); ?>
your recordset SQL would need to be:
WHERE r_categorie in(".implode(",", $_POST['result']).")