How do I check if a directory exists? "is_dir", "file_exists" or both?

$dirname = $_POST["search"];
$filename = "/folder/" . $dirname . "/";

if (!file_exists($filename)) {
    mkdir("folder/" . $dirname, 0777);
    echo "The directory $dirname was successfully created.";
    exit;
} else {
    echo "The directory $dirname exists.";
}

Both would return true on Unix systems - in Unix everything is a file, including directories. But to test if that name is taken, you should check both. There might be a regular file named 'foo', which would prevent you from creating a directory name 'foo'.