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Get the most recently updated file in PHP
file_12.css
file_34.css
file_14.css

int filemtime ( string $filename )

string readdir ([ resource $dir_handle ] )

array scandir ( string $directory [, int $sorting_order = SCANDIR_SORT_ASCENDING [, resource $context ]] )

<?php

$files = array();

if ($handle = opendir('/path/to/files')) {
    while (false !== ($file = readdir($handle))) {
        if (is_file($file)) {
            $modified = filemtime($file);
            $files[$modified] = $file;
        }
    }
    closedir($handle);
}

krsort($files);

$last_modified_file = array_shift($files);

function cmp($a, $b)
{
if(!is_file($b) || !strpos('file_', $b)) return -1;
$a = filemtime($a);
$b = filemtime($b);
return  ($a == $b) ? 0 :( ($a > $b) ? -1 : 1 );
}
usort($handle = scandir('/path/to/files'), "cmp");
$file = $handle[0];
unset($handle);