When I am calling the images of the contents in the website which is developped by PHP, there is no problem to display them. But i check the Access Logs of site, Only Firefox GETs the page TWO times. Other browsers GET the page for one time, but Firefox GETs two times.
Yes it was a problem for development. I made the headers but still not worked. Then i tried different header, now problem solved... may be needed in the future, the correct header codes like this:
// Last Modified date for caching
$file_last_modified = filemtime($filename);
// Expires in two months
$expires = time() + (60 * 24 * 60 * 60);
header("Expires: " . gmdate("D, d M Y H:i:s", $expires) . " GMT");
header("Last-Modified: " . gmdate("D, d M Y H:i:s", $file_last_modified) . " GMT");