jQuery DataTables: Uncaught TypeError: Cannot read property 'mData' of undefined

Why I'm getting error

Uncaught TypeError: Cannot read property 'mData' of undefined

I respect the DataTables requirements (and I also read another topics about my error and I respect every answer and solution). Please help me.

Here is my php code:

<table class="table table-striped table-bordered table-hover" id="sample_1">
        <thead>
            <tr>
                <th class="table-checkbox">
                    <input type="checkbox" class="group-checkable" data-set="#sample_1 .checkboxes"/>
                </th>
                <th>
                     Utilizator
                </th>
                <th>
                     Nume complet
                </th>
                <th>
                     Clasa
                </th>
                <th>
                     Functia
                </th>
                <th>
                     E-Mail
                </th>
                <th>
                     Ultima logare
                </th>
            </tr>
        </thead>
        <tbody>
            <?
               foreach($data["users"] as $student)
               {
            ?>
            <tr class="odd gradeX">
                <td>
                    <input type="checkbox" class="checkboxes" value="1"/>
                </td>
                <td>
                     <? echo $student["username"]; ?>
                </td>
                <td>
                     <? echo " ".$student["last_name"]." ".$student["first_name"].""; ?>
                </td>
                <td>
                     <? echo getclass($student["class"]); ?>
                </td>
                <td>
                     <?
                     $functie = 0;
                     if($student["role"] == 1)
                     {
                         $functie = 1;
                         echo "Administrator site";
                     }
                     if($student["fctsc"])
                     {
                         $functie = 1;
                         echo "Director";
                     }
                     if($student["diriginte"])
                     {
                         $functie = 1;
                         echo "Diriginte";
                     }
                     if($student["profesor"])
                     {
                         $functie = 1;
                         echo "Profesor";
                     }
                     if($functie == 0)
                         echo "Elev";
                     ?>
                </td>
                <td>
                    <a href=""email"]; ?>">
                        <? echo $student["email"]; ?>
                    </a>
                </td>
                <td class="center">
                     <? echo $student["lastlogin"]; ?>
                </td>
            </tr>
            <?
               }
            ?>
        </tbody>
    </table>


ANSWERS:


check weather or not all the php echo's are actually outputting something that is not 'NULL' on all rows as DataTables will treat any cell with just NULL as content as non existant and this can also cause that error.
try avoiding direct php output to html table.



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