Programmatic Context Level Settings

$MySetting = $modx->newObject('modContextSetting');
$MySetting->set('context_key', 'web');
$MySetting->set('key', 'mykey');
$MySetting->set('value', 'my_value');
$MySetting->set('xtype', 'textfield');
$MySetting->set('namespace', 'my_namespace');
$MySetting->set('area', 'MyArea');
$MySetting->save();
// Clear the cache:
$cacheRefreshOptions =  array( 'context_settings' => array() );
$modx->cacheManager->refresh($cacheRefreshOptions);

$cs = $modx->getObject('modContextSetting', 
    array(
        'context_key' => 'web', 
        'key' => 'someKey
    ),
);
if ($cs) {
  $cs->set('value', 'someValue');
  $cs->save();
  /* clear context_setting cache here if desired */
}