PHP explode() -> Keine Ausgabe?

<table class="table table-striped">
          <thead>
            <tr>
              <th>Monat</th>
              <th>Arbeitsstunden</th>
              <th>Lohn</th>
            </tr>
          </thead>
          <tbody>
            <?php
              $query = mysql_query("SELECT * FROM worktime WHERE ownerid='$uid'");
              while($row = mysql_fetch_array($query)){
                $mon = explode(".", $row['workdate']);
                echo "<tr>";
                echo "<td>" . $mon[1] . "/" . $mon[2] . "</td>";
                echo "<td>Platzhalter</td>";
                echo "<td>Platzhalter</td>";
                echo "</tr>";
              }
            ?>
          </tbody>
        </table>

<form method="get" action="payments.php">
       <div class="input-group" style="width: 80%; float:left;">
        <span class="input-group-addon">Arbeitsplatz:</span>
        <select name="workplace" class="form-control input-sm">
          <?php
            $query = mysql_query("SELECT * FROM workplace WHERE ownerid='$uid'");
            while($row = mysql_fetch_array($query)){
              echo '<option value="' . $row['id'] . '">' . $row['name'] . '</option>';
            }
          ?>
        </select>
      </div>


      <div class="input-group" style="width:19%; margin-left:1%; float:right;">
        <button class="btn btn-primary btn-sm" style="width:100%;" type="submit">Auswählen</button>
      </div>
    </form>