We can process multiple dropdown select using array concept.
Please refer the below example code.
<?php
if(isset($_POST['submit']))
{
$product=$_POST['product'];
//print_r($product);
$count=count($product);
for($i=0;$i<$count;$i++){
echo $product[$i]."<br>";
}
}
?>
<form action="#" method="post">
<select name="product[]" multiple="multiple">
<option value="product1">product1</option>
<option value="product2">product2</option>
<option value="product3">product3</option>
</select>
<input name="submit" type="submit" />
</form>
