Basic script to give you the idea. Push all the files with their times into an array, sort it by descending time order and walk trough. if($count > 10) says when the deletion should start, i.e. currently it keeps the newest 10.
<?php
$directory = ".";
$files = array();
foreach(scandir($directory) as $file){
if(is_file($file)) {
//get all the files
$files[$file] = filemtime($file);
}
}
//sort descending by filemtime;
arsort($files);
$count = 1;
foreach ($files as $file => $time){
if($count > 10){
unlink($file);
}
$count++;
}