Why is mime_content_type() deprecated in PHP?

The method is not deprecated!

It once was incorrectly marked as deprecated in the manual, but it has been fixed https://bugs.php.net/bug.php?id=71367 on the 14th of January 2016. However, at the moment, it is still incorrectly marked deprecated in the German, Spanish and Chinese manual.

Feel free to use mime_content_type() whenever you like :).

I guess it's because Fileinfo can return more information about files.

EDIT: Here is a replacement hack:

function _mime_content_type($filename) {
    $result = new finfo();

    if (is_resource($result) === true) {
        return $result->file($filename, FILEINFO_MIME_TYPE);
    }

    return false;
}

Another way is to pass to the constructor constant FILEINFO_MIME.

$finfo = new finfo(FILEINFO_MIME);
$type  = $finfo->file('path/filename');