- array_map: documentation ( source)
- var_dump: documentation ( source)
<?php
$path = 'C:/123/';
$list = array('f1.txt', 'f2.txt', 'f3.txt', 'f4.txt');
$mapping = array_map(function($i, $item) use ($path) { return $path ? $path . $item : $item; }, $list);
var_dump($mapping);