multiselect and php

[The Hobo]

How can I get the value of each item selected? This only gives me the last item selected:

if ($action == "showdata") {
    echo $cats;
} else {
    showform();
}

function showform() {

echo '
<form action="work.php?action=showdata" method="post">

    <select name="cats" size="5" multiple>
    <option value="1">Cow</option>
    <option value="2">Eats</option>
    <option value="3">Blue</option>
    <option value="4">Grass</option>
    <option value="5">Y\'all</option></select>
    
    <br><input type="Submit"></form>';
    
}