<?php

$filenames = array(
    'Vacation (1).png',
    'Vacation (Me and Mom) (2).png',
    'Vacation (5) (3).png',
);

foreach ( $filenames as $filename ) {
    if ( preg_match( '/^.*\((\d+)\)/s', $filename, $matches ) ) {
        var_dump($matches);
    } else {
    }
}