Actually, I think I found how to set the 404:

location ~ \.php$ {
if (!-f $document_root/$fastcgi_script_name) {
log_not_found off;
return 404;
}

include /etc/nginx/fastcgi_params;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
fastcgi_intercept_errors off;
fastcgi_pass php1.local.pagelines.com:9000;
}

But I can't use log_not_found off, getting an error about the ability to use this in the location. Basically, I don't want an error logged if somebody types:

foobar.mydomain.com