How to reduce code duplication - php

Question

As I'm not familiar with PHP I wonde how do I reduce code duplication here? Both methods are doing exactly the same thing here... except the part where the string is extracted (filemtime and basename) and joined.
private function modified_hash( $files ) {
$joined = "";
foreach ( $files as $file ) {
$joined .= filemtime( $file );
}
return $this->checksum( $joined );
}
private function filename_hash( $files ) {
$joined = "";
foreach ( $files as $file ) {
$joined .= basename( $file );
}
return $this->checksum( $joined );
}

Answer 1

Instead of two functions, declare a unified function with an argument for a crucial callback/function name $func_name:
/**
* Gets joined files hash
*
* #param $files an array of file paths
* #param $func_name callback name
* #return mixed
*/
private function getFilesHash($files, callable $func_name) {
$joined = "";
foreach ($files as $file) {
$joined .= call_user_func($func_name, $file);
}
return $this->checksum($joined);
}
Usage:
$fileHash = getFilesHash($files, 'basename');
Used functions:
call_user_func

Answer 2

I guess my version is way bigger then Romans, but as an OOP problem I think this is a possible solution too:
<?php
interface HashInterface
{
public function hash();
}
class ModifiedHash implements HashInterface
{
public function hash($file)
{
return filemtime($file);
}
}
class FileNameHash implements HashInterface
{
public function hash($file)
{
return basename($file);
}
}
class SomeClient
{
private $hashType;
public function setHashType(HashInterface $hashType)
{
$this->hashType = $hashType;
}
private function doHash( $files ) {
$joined = "";
foreach ( $files as $file ) {
$joined .= $this->hashType->hash( $file );
}
return $this->checksum( $joined );
}
}
$client = new SomeClient();
$files = ???;
// Want a ModifiedHash?
$client->setHashType(new ModifiedHash());
$data = $client->doHash($files);
// Want a FileNameHash?
$client->setHashType(new FileNameHash());
$data = $client->doHash($files);
Sorry for the confusing class or method names. I hope you got the idea.