Well, for [man]strtotime/man you need an RFC compliant date format. Last time I checked, 090807 was not one of them.
GNU :: Date Input Formats
If you wanted that to turn into 2009-08-07, you could do something like:
<?php
$date = 090807;
$parts = explode('-', chunk_split($date, 2, '-'));
$date = '20' . $parts[0] . '-' . $parts[1] . '-' . $parts[2];
echo $date;
But you're better off specifying dates in some GNU format. Makes life much easier.