How can I get a field value of a node using PHP?
The easiest non Entity API way of doing it is to use field_get_items():

// Assuming you have the node object and assuming it is a single value
// field or you just want the first value of a multi value field.
$username_value = '';
if ($items = field_get_items('node', $node, 'field_user_name')) {
  $username_value = $items[0]['value'];
}