Description:
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for($i=12;$i>=0;$i--){
  $dt 	= mktime(0, 0, 0, date("m")-$i, '1', date("Y"));
  $mois[] = 	date('M-y',$dt) .';'.
	 	utf8_encode(strftime("%h %g",$dt)) .';'.
	 	$dt;
	 	}
var_dump($mois);

Test script:
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Today january 2016 the last 2 lines are:

array(3) { [0]=> string(6) "Dec-15" [1]=> string(8) "déc. 15" [2]=> string(10) "1448924400" }

array(3) { [0]=> string(6) "Jan-16" [1]=> string(8) "janv. 15" [2]=> string(10) "1451602800" }

See [1] of the last line... why janv. 15 and not janv. 16 ??

Expected result:
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array(3) { [0]=> string(6) "Jan-16" [1]=> string(8) "janv. 16" [2]=> string(10) "1451602800" }

Actual result:
--------------
array(3) { [0]=> string(6) "Jan-16" [1]=> string(8) "janv. 15" [2]=> string(10) "1451602800" }