如何确保无法访问处理ajax请求的PHP脚本并对用户进行身份验证？

I have an jquery ajax request that calls a PHP script that will send an email. This email is happening from the admin, so a user must be authenticated in order to be able to do this. I've got two questions:

1. How can I lock this PHP file down from somebody being able to go directly to the path in the browser and keep submitting it?
2. How do I only run the file if the user is authenticated?

PHP:

$emailer = new GiftCardEmailer();
$emailer->SendGiftCardEmail($gift_card);

jQuery:

 $(document).ready(function() {                
     var status = $('p#status');
     status.hide();

     $('#sendemail').click(function() {                   
          $.ajax({
                    url: 'mail-handler.php',
                    beforeSend: function() {
                        status.fadeIn();
                        status.html('<img src="images/ajax-loader.gif" />');  
                    },                        
                    success: function( data ) {
                        if (console && console.log){
                            console.log( 'Sample of data:', data.slice(0,100) );
                        }                            
                        status.html('Email Sent Successfully.');   
                        setTimeout(function() {
                            status.fadeOut(); 
                        }, 4000);                            
                    }
                });
            });
        });

One approach would be to check for a valid session at the head of the file containing the actual mail code. If no session exists then simply terminate the script.

Check out ajax and security, might be helpful.

I do all of mine with session_start() at the top of a php page and then check for my specific session variables for authentication.

No session, no access.