test/index.php

<?php

$date = date('Ym');
$arr = array();
$arr[] = $date;
for($i=1;$i<6;$i++){
    $arr[] = date("Ym",strtotime("$date - $i month "));
}
var_dump($arr[0]);
echo "<br />";
var_dump($arr[1]);
echo "<br />";
var_dump($arr[2]);
echo "<br />";
var_dump($arr[3]);
echo "<br />";
var_dump($arr[4]);
echo "<br />";
var_dump($arr[5]);
echo "<br />";
echo "<br />";
echo "<br />";
echo "<br />";
echo "<br />";
echo "<br />";
echo "<br />";
echo "<br />";
var_dump($date );
?>