I want to be able to break out of my PHP script that is called with jQuery Ajax. How can I return an error back to jQuery to display to the user?

PHP: 

```
if($gift_card != null)
{
echo "not null";            
}
else 
{
echo "null";    
exit();         
}

```

jQuery:

```
error: function(jqXHR, textStatus, errorThrown) {
                        status.html("There was an error processing the request. " + errorThrown);  
                        setTimeout(function() {                                
                            status.fadeOut(); 
                        }, 4000);                            
                    },
                    success: function( data ) {
                        if (console && console.log){
                            console.log( 'Sample of data:', data);
                        }                            
                        status.html('Email Sent Successfully.');   
                        setTimeout(function() {
                            status.fadeOut(); 
                        }, 4000); 

```

If I echo null and exit, it is still coming back as success. Do I just have to check the data in success and see what is returned and then go from there?