为PHP添加的DIV标记添加边距

I want to add margin to this <div>:

$notification_list .= "
    <div class='FriendRdiv'>
        <a href='main.php?u=".$initiator."'>
            <img src='../img/".$pr."' id='FriendRPP'>
        </a>
        <a id='FriendRfrom'>".$initiator." sent you a friend request</a>
    </div>
    <form class='FriendRForm Rejecat' onsubmit='return false'>
        <input type='submit' value='Reject' id='Reject'>
    </form>         
    <form class='FriendRForm' onsubmit='return false'>
         <input type='submit' value='Accept' id='Accept'>
    </form>";

I don't know how many notifications I will get, so I want each <div> to have a bigger margin than one before it.

you can use style attribute

<div class='FriendRdiv' style="margin: 10px;">

or you can use css for that in css file

.FriendRdiv{
   margin: 10px;
}