PHP/MySQL: updating every row with a random value - foreach not working (updating all rows with same value)

I have around 1000 records in my MySQL database. I recently added a new column called UniqueKey and I want to update all rows in my table with a random, unique key.

This is what I got:

$updatequery = "SELECT * FROM pictures";
    $resultt = $conn->query($updatequery);

    while ($roww = $resultt->fetch_array()) {
        $rowws[] = $roww;
    }

    foreach($rowws as &$roww) {
        $UniqueKey = substr(str_shuffle('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'), 0, 6);
        $sqlupdate = "UPDATE pictures SET UniqueKey = '$UniqueKey'";
        mysqli_query($conn, $sqlupdate);

    }

The problem here is as follows. It DOES create a UniqueKey, but it uploads this new random value to ALL my rows. How can I do this?:

• generate random key by $UniqyeKey
• Insert this key in the first row
• regenerate a new key and insert this in the next row
• etc etc

Thanks in advance! Have been trying to fix this for hours..