Distinct Items in an Array

I have a CSV type file that has a date field in it. The format is "MM-DD-YYYY". I would like to break this up and push into an array the first instance of a changed Month. For instance, if there is 14 instances of "Feb", 21 instances of Mar and 4 instance of Jun, I only want the array to have Feb in the first position and then have a NULL character for the next 13 instances, then Mar and then 20 NULL characters, then Jun and 3 NULL characters. This is the code I wrote and it gives me the opposite effect. What I get is the first 13 instances in the array are blank and the last one is Feb etc...I want the opposite, I need the first instance to be Feb, and the rest to be blank until the new month comes along. $Data = fopen("thefile.txt","r"); $LoopCount =1; while ($Line = fgetcsv ($Data, 1000, ",")) { if($LoopCount != 1) { //Date,Open,High,Low,Close,Volume array_push($Close, $Line[4]); array_push($Open, $Line[1]); list($DDate, $MDate, $YDate) = split("-", $Line[0]); if($OldMDate != $MDate) { array_push($Date, $MDate); } else { array_push($Date, ""); } $OldMDate = $MDate; } $LoopCount++; }