To implement Combo box also known as drop-down list, you should use <select> tag
<label for="cars">Choose a car:</label> <select name="cars" id="cars"> <option value="volvo">Volvo</option> <option value="saab">Saab</option> <option value="mercedes">Mercedes</option> <option value="audi">Audi</option> </select>
If you want to do something when user select something,
<select onchange="doSomething();" onfocus="this.selectedIndex = -1;"> <option>A</option> <option>B</option> <option>C</option> </select>
In case that you use PHP combined with select above, below example will be helpful.
<?php
$country_list = [
"LA",
"USA",
"AUS",
"BRA",
"CAN",
"CHN",
"EU",
"IND",
"JPN",
"RUS",
"SEA",
"TUR"
];
if (strlen($c)<1) $c="USA";
echo '<label for="c">Choose a region:</label><select name="c" id="c">';
foreach($country_list as $v)
{
echo "<option value=\"{$v}\">{$v}</option>";
}
echo "</select>";
?>
Below example changes the URL dynamically by the selected country above.
<script>
function changeRegion()
{
self.location = "<?php echo "{$PHP_SELF}?m={$m}&c="; ?>" + document.getElementById('c').value;
}
</script><?php
$country_list = [
"LA",
"USA",
"AUS",
"BRA",
"CAN",
"CHN",
"EU",
"IND",
"JPN",
"RUS",
"SEA",
"TUR"
];
if (strlen($c)<1) $c="USA"; // BRA"; // need to get from parameters
echo '<label for="c">Choose a region:</label><select name="c" id="c" onchange="changeRegion();" onfocus="this.selectedIndex = -1;">';
foreach($country_list as $v)
{
echo "<option value=\"{$v}\">{$v}</option>";
}
echo "</select>";
?>