How to make a website in PHP work both in HTTP and HTTPS?

Use relative paths. If you are pointing to something that is on the same site as yours, then you should not be using http://

If for some reason you still need to have http:// then just switch them all to https://. An http:// will never complain because it is pointing to https:// stuff, but an https:// page will complain if it is pointing to non-https stuff.

If you are pointing to content outside of your control, on another site for example, then you need to hope that you can get at that content via https instead. If you can't, then you're hosed and you either need to live with the error, get the content from somewhere else, or proxy the content through your own https connection.

To complement @drew010 's answer, you could use other domains and still refer to the current protocol with //, something like:

<img src="/pics/home.png" />
<img src="//my-cdn.com/pics/info.png" />

The latter example will point to https://.. from https://your-site.com and http://... from http://your-site.com.

the best practice would be either using relative path rather than absolute but sometimes absolute is a better option so you can do the following :

as I can imagine you have a file called config.php or common.php (a file that stores your common used vars and you include it in every page), so put this code there :

function selfURL() {
$s = empty($_SERVER["HTTPS"]) ? '' 
    : ($_SERVER["HTTPS"] == "on") ? "s" : "";
$protocol = strleft(strtolower($_SERVER["SERVER_PROTOCOL"]), "/").$s;
$port = ($_SERVER["SERVER_PORT"] == "80") ? "" : (":".$_SERVER["SERVER_PORT"]);
return $protocol."://".$_SERVER['SERVER_NAME'].$port.$_SERVER['REQUEST_URI'];
}

function strleft($s1, $s2) {
return substr($s1, 0, strpos($s1, $s2));
}

and then you can assign a var called $http to get the value of the function like : $http = selfURL(); and then whenever you want to include anything like images, css, etc do something like :
<img src="<?=$http?>images/sample.png" />

this method is reliable as it works in any situation.