With PHP code and styling

PHP

  $CheckboxSetting1 = False;
  $TextboxSetting1 = "";
  $TextareaSetting1 = "";
  $SelectBoxSettingA = 0;
  if (isset($_POST['form_submit_id']) && ($_POST['form_submit_id'] == 'admin_store_settings'))
  {
    //------------------------------
    //----- STORE NEW SETTINGS -----
    //------------------------------
   
    //CHECK THE FORM NONCE FIELD IS VALID
    if (
      (!isset( $_POST['my_form_nonce'])) ||
      (wp_verify_nonce($_POST['my_form_nonce'], 'my_form') !== 1)       //1=nonce created within last 12 hours
    )
    {
      wp_redirect( home_url( '/' ) );
      die;
    }

    $CheckboxSetting1 = False;
    if(isset($_POST['CheckboxSetting1']) && $_POST['CheckboxSetting1'] == 'Yes')
      $CheckboxSetting1 = True;
    
    $TextboxSetting1 = "";
    if(isset($_POST['TextboxSetting1']))
    {
      $TextboxSetting1 = trim($_POST['TextboxSetting1']);
      $TextboxSetting1 = stripslashes($TextboxSetting1);
      $TextboxSetting1 = htmlspecialchars($TextboxSetting1, ENT_QUOTES);
    }
    
    $TextareaSetting1 = "";
    if(isset($_POST['TextareaSetting1']))
    {
      $TextareaSetting1 = trim($_POST['TextareaSetting1']);
      $TextareaSetting1 = stripslashes($TextareaSetting1);
      $TextareaSetting1 = htmlspecialchars($TextareaSetting1, ENT_QUOTES);
    }
    
    $SelectBoxSettingA = 0;
    if(isset($_POST['SelectBoxSettingA']))
      $SelectBoxSettingA = intval($_POST['SelectBoxSettingA']);
  }

  $CheckboxSetting1_Checked = "";
  if ($CheckboxSetting1)
    $CheckboxSetting1_Checked = "checked";

  $SelectBoxSettingA = 0;
  $SelectBoxSettingA_Selected = array();
  $SelectBoxSettingA_Selected = array_fill(0, 3, "");		//Fill an array from index 0, 3 items long, with default value ""
  $SelectBoxSettingA_Selected[($SelectBoxSettingA)] = "selected ";


  //-----------------------
  //-----------------------
  //----- HTML OUTPUT -----
  //-----------------------
  //-----------------------
  $HtmlOutput = '';
  
  $OurNonceField = wp_nonce_field( 'my_form', 'my_form_nonce', true, false );   //Used to validate that the contents of the form request came from the current site and not somewhere else

 
  //(No form action="" specified so form will submit to same URI as this page)
  $HtmlOutput .= <<<_END
    <h3>Settings</h3>
    <form method="POST">
      <input type="hidden" name="form_submit_id" value="admin_store_settings" />
      $OurNonceField

      <input type="checkbox" name="CheckboxSetting1" $CheckboxSetting1_Checked value="Yes" /><label>Checkbox Setting 1</label>

      <label>Textbox Setting 1:</label>
      <input type="text" name="TextboxSetting1" style="width:100%; " value="$TextboxSetting1" />

      <label>Textarea Setting 1:</label>
      <textarea name="TextareaSetting1" style="width:100%; height:60px;">$TextareaSetting1</textarea>

      <label>Select Box Setting 1:</label>
      <select name="SelectBoxSettingA" style="width:200px">
        <option value="0" $SelectBoxSettingA_Selected[0]>My Option 0</option>
        <option value="1" $SelectBoxSettingA_Selected[1]>My Option 10</option>
        <option value="2" $SelectBoxSettingA_Selected[2]>My Option 20</option>
        <option value="3" $SelectBoxSettingA_Selected[3]>My Option 30</option>
      </select>

      <input type="submit" class="AdminButton" value="Save Settings" />
    </form>
_END;

CSS

/*-----------------*/
/*-----------------*/
/*----- ADMIN -----*/
/*-----------------*/
/*-----------------*/

.AdminPage {

}

/*----- FORM FIELDS -----*/
.AdminPage label {
  display: block;    /*Cause newline after*/
}

.AdminPage input[type="text"] {
  margin-bottom: 10px;
  display: block;    /*Cause newline after*/
  font-size: 16px;   /*min 16px for safari*/
}

.AdminPage input[type="file"] {
  margin-bottom: 10px;
  display: block;    /*Cause newline after*/
  font-size: 16px;   /*min 16px for safari*/
}

.AdminPage textarea {
  margin-bottom: 10px;
  display: block;    /*Cause newline after*/
  font-size: 16px;   /*min 16px for safari*/
}

.AdminPage input[type="checkbox"] {
  display: inline-block;
}
.AdminPage input[type="checkbox"] + label {
  display: inline-block;
  margin-bottom: 10px;
}

.AdminPage select {
  border: 1px solid #f6f6f6;
  margin-bottom: 10px;
  padding: 2px;
  display: block;    /*Cause newline after*/
  font-size: 16px;   /*min 16px for safari*/
}


/*----- BUTTON -----*/
a.AdminButton, a.AdminButton:hover, a.AdminButton:visited,
 input[type="submit"].AdminButton {
  background-color: #007cba;
  border: 1px solid #007cba;
  border-radius: 3px 3px 3px 3px;
  min-height: 36px;
  min-width: 80px;
  margin-top: 8px;
  margin-right: 8px;
  margin-left: 0px;
  margin-bottom: 8px;
  padding-top: 6px;
  padding-right: 6px;
  padding-left: 6px;
  padding-bottom: 6px;
  font-size: 13px;
  font-weight: normal;
  line-height: 40px;
  color: #fff;
  text-align: center;
  text-decoration: none;
  cursor: pointer;
}

 input[type="submit"].AdminButton {
   line-height: 13px;
}

/*----- Messages -----*/
.AdminPageMessage {
  margin-top: 12px;
  margin-bottom: 12px;
  text-align: left;
  font-weight: bold;
  color: #00B35F;
}