Get array of months between 2 dates

Im trying to get a list of months between 2 dates, and came up with:-

        function echoDate( $start, $end ){

            $current = $start;
            $ret = array();

            while( $current<$end ){
                $next = date('Y-M-01', $current) . "+1 month";
                $current = strtotime($next);
                $ret[] = date("Y-m",$current);
            }

            return $ret;
        }

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So, if I ran:-

echoDate(strtotime("2021-09-06"), strtotime("2021-12-26"));

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I should get an array of:-

2021-09
2021-10
2021-11
2021-22

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But instead I get an array of:-

2021-10
2021-11
2021-12
2022-01

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Which I dont understand of where its getting these values from.

Any ideas?