compare multiple select using an array [] as the select name and a scalar as the select name. The first returns the multiple values chosen. the latter only returns one of the values selected.

compare_multiple.php

<h1>Test what gets returned from &lt;select multiple&gt; when using array and scalar</h1>
<?php
if (count($_POST)>0) {
  echo "<h2>POST</h2><pre><code>\n";
  var_dump($_POST);
  echo "</code></pre>\n";
}
?>

<form method="POST">

<h3>Select multiple options, send as an array</h3>
<select name="group_one[]" multiple>
  <option value="1">one</option>
  <option value="2">two</option>
  <option value="3">three</option>
  <option value="4">four</option>
</select>


<h3>Select multiple options, send as a single value</h3>
<select name="group_two" multiple>
  <option value="1">one</option>
  <option value="2">two</option>
  <option value="3">three</option>
  <option value="4">four</option>
</select>


<input type="submit" name="submit" value="submit" />

</form>

output

In this case, I selected the same items in each <select> field.

POST

array(3) {
  ["group_one"]=>
  array(3) {
    [0]=>
    string(1) "1"
    [1]=>
    string(1) "2"
    [2]=>
    string(1) "4"
  }
  ["group_two"]=>
  string(1) "4"
  ["submit"]=>
  string(6) "submit"
}