hide or show textbox based on the dropdown selection in php

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PHP

 <?php
                class combobox extends Status
                {
                     function showTextBox()     
                     {
                         if (Status.$id != 3)
                         {
                              echo '<td><span class="required">*</span> Reward (<span class="style1">Optional</span>):</td>
                              <td><input name="reward" type="text" id="reward" /></td>';
                         }       
                     }
                     
                }
                   $combo = new combobox();
                   $combo.showTextBox();
//              ?>

Good Day all

The above code is suppose to hide or show a textbox based on the selection made in the dropdown. however i am having an error saying the function is not define. please help
thanks in advance
Regards