<?php
$files = array('images/1086_002.jpg', 'images/1086_0021.jpg', 'images/1086_00211.jpg', 'images/1086_002111.jpg');
$fullpath = 'images/1086_002.jpg';
$additional = '1';
// replaced file_exists with in_array for this example
while (in_array($fullpath, $files)) {
$info = pathinfo($fullpath);
$fullpath = $info['dirname'] . '/'
. $info['filename'] . $additional
. '.' . $info['extension'];
}
var_dump($fullpath);