Sort An Array By Day, User Same Or Page Id

PHP script to sort the following array by the day (page_id) and username.

<?php
$arra[0]["transaction_id"] = "2025731470";
$arra[1]["transaction_id"] = "2025731450";
$arra[2]["transaction_id"] = "1025731456";
$arra[3]["transaction_id"] = "1025731460";

$arra[0]["user_name"] = "Sana";
$arra[1]["user_name"] = "Illiya";
$arra[2]["user_name"] = "Robin";
$arra[3]["user_name"] = "Samantha";

//convert timestamp to date
function convert_timestamp($timestamp){
    $limit=date("U");
    $limiting=$timestamp-$limit;
    return date ("Ymd", mktime (0,0,$limiting));
}

//comparison function
function cmp ($a, $b) {
    $l=convert_timestamp($a["transaction_id"]);
    $k=convert_timestamp($b["transaction_id"]);

    if($k==$l){
        return strcmp($a["user_name"], $b["user_name"]);
    }
else{
        return strcmp($k, $l);
    }
}

//sort array
usort($arra, "cmp");

//print sorted info
while (list ($key, $value) = each ($arra))
{
    echo "\$arra[$key]: ";
    echo $value["transaction_id"];
    echo " user_name: ";
    echo $value["user_name"];
    echo "\n";
}
?>