php函数调用中的下划线

I know that underscores in function names in PHP is used to "implicitly" denote that they are supposed to be private...but I just saw this code:

class DatabaseConnection
{
  public static function get()
  {
    static $db = null;
    if ( $db == null )
      $db = new DatabaseConnection();
    return $db;
  }

  private $_handle = null;

  private function __construct()
  {
    $dsn = 'mysql://root:password@localhost/photos';
    $this->_handle =& DB::Connect( $dsn, array() );
  }

  public function handle()
  {
    return $this->_handle;
  }
}

print( "Handle = ".DatabaseConnection::get()->handle()."
" );
print( "Handle = ".DatabaseConnection::get()->handle()."
" );

in this code, what does it mean to have underscores in variables?

It's kind of the same for methods and properties : the convention is the same : having a name that starts by one underscore generally means they are to be considered as private/protected.

(Of course, it's not the same with methods which have a name that starts by two underscore : those are magic methods, and two underscore should not be used for your "normal" method names)

Ahh __construct is a special method. It is PHP way of saying that is the constructor. They stole that from Python probably where those are magic methods.

When a class is instantiated the constructor is called automatically.

so if you create the object myconnection = DatabaseConnection() it will automatically call:

$dsn = 'mysql://root:password@localhost/photos';
$this->_handle =& DB::Connect( $dsn, array() );

... the code in the constructor. So then when you call get() later (be careful it is static) it will have a connection.

I sometimes move that code into a different method, say connect() and call that in the constructor, so I can do it either way. I imagine the real thing that is tripping you up is the 'static' method get. Those do not require a fully constructed object to be called.

The convention is usually this:

• one underscore is usually used for protected/private methods and variables, so that they stick out
• two underscores are reserved for "magic" methods like __construct, __get, __set etc. which have special meaning in the language, so you should not use it unless you writing a known magic method.

This said, the language does not assign any special meaning to one-underscore names, it's just a convention - so you don't have to make it private, and some people with backgrounds in other languages for example mark all properties with underscore to distinguish them from methods.

Properties (variables) have visibility just like methods (functions).