You can easily check mime type of an internet resource using this code :
<?php
function getUrlMimeType($url) {
$buffer = file_get_contents($url);
$finfo = new finfo(FILEINFO_MIME_TYPE);
return $finfo->buffer($buffer);
}
?>
I'm using it to detect if an url given by a user is a HTML page (so I do some stuff with the HTML) or a file on Internet (so I show an icon accordingly to the mime type).