<?php

$file = "myimage.jpg";
$fileBase = substr($file, 0, strlen($file) - strlen(pathinfo($file, PATHINFO_EXTENSION)) - 1);
$newname = $fileBase.md5(mktime()).'.'.pathinfo($file, PATHINFO_EXTENSION);
echo $newname;

?>

Output:
myimage78e59e6cb44fb6cc4d68f562d9ea353d.jpg