Understanding the meaning of the existence of interface in PHP interface

Maybe we all understand these, as do not understand I guess the meaning of this interface, he is in order to call the next time to call the implementation of the interface in the class of the content, good around the mouth ah, write an example to stay for later see
pay.php

Copy Code code as follows:
Interface Ipay
{
function Withmoney ();

function withinternet ();
}
Class Dmeng implements Ipay
{

function Withmoney ()
{
echo "Spend RMB to buy things";
}
function Withinternet ()
{
Return "Pay with net silver";
}
}

usei.php

Copy Code code as follows:
Include_once ' pay.php ';
Class Main
{
function Run ($VC)
{
$this->VC = $VC;
$this->vc->withinternet ();
echo "Yunxing";
}

}
$com = new Main ();
$com->run (new Dmeng);

That's the way it is, we annotate a method in the interface, and when we do it, it's useless.