Easily, at the top of the page where you wish to have the pop up appear execute the function
setcookie("cookie_name","data");
This will cause the cookie name 'cookie_name' to be put into the browsers cookies. Next time they goto this page (if the above function was ran) you'll have a variable defined within your php page automatically named 'cookie_name' in which you can just put a if statement around the popup code that ou print out to the browser.. i.e.
if(!$cookie_name)
{
setcookie("cookie_name","data");
// PRINT POPUP CODE
}
You should be aware though you cannot print out any information before setcookie because setcookie must be ran before the headers are outputted (the headers for php are outputted onn the first instance that any text is sent to the browser). There is a setting however you can change in php.ini to have php parse for setcookie or headers() functions and run those before it outputs the headers
