<select name="MySelectBox" style="width: 300px;">
	<option value="0" selected>Option 0</option>
	<option value="1" >Option 1</option>
</select>

Causing Form To Submit OnChange

 <select name="MySelectBox" onchange="this.form.submit()">

Loading With Selected Setting From PHP – Using an array

PHP

  //Create the select box values and select the currently selected value
  $MySelectBoxValues = array(
    1 => 'Current hour',
    2 => 'Last 2 hours',
    6 => 'Last 6 hours',
    12 => 'Last 12 hours',
    24 => 'Last 24 hours',
    168 => 'Last 7 days',
    744 => 'Last 31 days'
  );
  $MySelectBoxHtml = '';
  foreach ($MySelectBoxValues as $Value => $Label)
  {
    $Selected = ($ViewStatsNumOfHours == $Value) ? ' selected' : '';
    $MySelectBoxHtml .= "<option value=\"{$Value}\"{$Selected}>{$Label}</option>";
  }

HTML

  <select name="MySelectBox" style="width:300px;" >
    $MySelectBoxHtml
  </select>

Auto submit select box form example

PHP

  $MySelectBox = 24;
  if (isset($_POST['my_form_submit_id']) && ($_POST['my_form_submit_id'] == 'select_box_used'))
  {
    //----- SELECT BOX USED -----
    $MySelectBox = intval($_POST['MySelectBox']) ?? 24;

    echo "[$MySelectBox]";
  }

  //Create the select box values and select the currently selected value
  $MySelectBoxValues = array(
    1 => 'Current hour',
    2 => 'Last 2 hours',
    6 => 'Last 6 hours',
    12 => 'Last 12 hours',
    24 => 'Last 24 hours',
    168 => 'Last 7 days',
    744 => 'Last 31 days'
  );

  $MySelectBoxHtml = '';
  foreach ($MySelectBoxValues as $Value => $Label)
  {
    $Selected = ($MySelectBox == $Value) ? ' selected' : '';
    $MySelectBoxHtml .= "<option value=\"{$Value}\"{$Selected}>{$Label}</option>";
  }

HTML

  <form method="POST">

    <input type="hidden" name="my_form_submit_id" value="select_box_used" />

    <select name="MySelectBox" style="width: 300px;" onchange="this.form.submit()" >
      $MySelectBoxHtml
    </select>
  </form>

Doing specific things

Get selected item text

  <select id="SelectCategory" name="SelectCategory" style="width:100%; max-width:400px" onchange="javascript:PopulateCategoryType();" >
    ...
  </select>

  <script>
    function PopulateCategoryType() {
      var e = document.getElementById('SelectCategory');
      var result = e.options[e.selectedIndex].text;
      document.getElementById('ResourceCategoryType').value = result;
    }
    setTimeout(PopulateCategoryType(), 2500);
  </script>