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

I want to create a directory if it does not exist already.

Is using the is_dir function enough for that purpose?

if ( !is_dir( $dir ) ) {
    mkdir( $dir );       
}

Or should I combine is_dir with file_exists?

if ( !file_exists( $dir ) && !is_dir( $dir ) ) {
    mkdir( $dir );       
}