<?php

function cleanCache($pathToCacheFolder = null, $notAccessedInXDays = null){
    // uncomment the next lines if you want to stop the command if parameters are null
    // without showing any error
    /*
    if ($pathToCacheFolder === null || $notAccessedInXDays == null) {
        return;
    }
    */
    // throw exception if dir doesn't exist
    if ( ! is_dir($pathToCacheFolder)) {
        // if you don't want to show an error if pathToCacheFolder doesn't exist
        // just replace the next line with "return;"
        throw new RuntimeException("Folder '${pathToCacheFolder}' doesn't exist");
    }
    // create our file iterator for the given path
    $iterator = new FileSystemIterator($pathToCacheFolder);
    // ignore . and ..
    $iterator->setFlags(FileSystemIterator::SKIP_DOTS|FileSystemIterator::CURRENT_AS_PATHNAME);

    foreach ($iterator as $file) {
        // current time
        $now = new DateTime();

        // set file modified datetime
        $filemtime = new DateTime();
        $filemtime->setTimestamp(filemtime($file));

        // diff those two
        $diff = $now->diff($filemtime);

        $ext = pathinfo($file, PATHINFO_EXTENSION);

        // if file extension is one of these
        // and file wasn't modified since $notAccessedInXDays
        if (in_array($ext, array('gif', 'jpg', 'png')) && $diff->format("a") > $notAccessedInXDays) {
            // delete it
            unlink($file);
        }
    }
}

// usage example
// delete all jpg,png,gif files that weren't modified since 15 days
// inside "cache" dir
cleanCache('./cache', 15);