//open the box, that surrounds the messages
echo("<div class='dialogueBox'>");

while($row = mysqli_fetch_row($erg)) {
    //determine if the message is your own
    if($row[0] != $_SESSION["username"]) $ownmess = false;
    else                                 $ownmess = true;

    //begin div dependend on the sender of the message
    if($ownmess) echo ("<div class='dialogue own' >");
    else         echo ("<div class='dialogue other'>");

    //write Text
    echo($row[3] . "<br>");

    //write the timestamp as 'little' text (write blue, if it was read and its your own message)
    echo("<a class='little'");
        if($row[4] != 0 && $ownmess) echo(" style='color:Blue;'");
    echo(">" . $row[2]."</a>");

    //close div, and end message
    echo("</div> <br><br><br>");

}


//close big div, that surrounds all messages
echo("<br></div>");

.dialogueBox {
    width: 50%;
    margin-left: 25%;
    border: 1px solid Black;
    background-color: #BBAA88;
    max-height: 500px;
    overflow: auto;
}

.dialogue {
    border: none;
    margin: 5px;
    border-radius: 5px;
    padding: 3px;
    font-size: 15;
    white-space: pre-wrap;
}

.own {
    float: right;
    background-color: #9aaa63;
}

.other {
    float: left;
    background-color: #cec178;
}

.little {
    font-size: 10px;
}