To send PHP GET parameters to an iframe, you can simply append the parameters to the URL of the iframe's src attribute. For example, if you have an iframe with the src attribute set to "example.com/iframe.php", you can add GET parameters by modifying the URL to "example.com/iframe.php?param1=value1&param2=value2". This way, when the iframe loads, it will receive the parameters as part of the URL and you can access them in your PHP code within the iframe file.