Checking Selection Lists

 isset($_POST['listname']) && $_POST['listname'] != '') 

When it comes to validating a selection list, the approach depends on the type of list:

• If it is a list in which only one element may be selected, the list is considered to be filled out incorrectly if

• No option in the list is selected

• The selected option has an empty string as a value

• If it is a list in which multiple elements may be selected, the list is considered to be filled out incorrectly if

• No option in the list is selected

• All selected options have an empty string as a value

Options with empty strings as values come into play if the list contains functionless dummy entries that have captions such as "Please choose from list." These list options must not receive a value other than "", so that the validation algorithm can distinguish these entries from reasonable ones.

The listing at the beginning of this phrase validates a single selection listagain including PHP code to prefill the list.

Validating Lists (mandatory-list.php; excerpt)

 <?php   if (isset($_POST['Submit']) &&       isset($_POST['listname']) &&       $_POST['listname'] != '') {     echo '<h1>Thank you for filling out this       form!</h1>';   } else { ?>   <form method="post" action="<?php echo      htmlspecialchars($_SERVER['PHP_SELF']); ?>">     <select name="listname">       <option value="">Please select from         list</option>       <option value="php3"<?php   if (isset($_POST['listname'])    && $_POST['listname'] == 'php3') {     echo ' selected="selected"';   }       ?>>PHP 3</option>       <option value="php4"<?php   if (isset($_POST['listname'])    && $_POST['listname'] == 'php4') {     echo ' selected="selected"';   }       ?>>PHP 4</option>       <option value="php5"<?php   if (isset($_POST['listname'])    && $_POST['listname'] == 'php5') {     echo ' selected="selected"';   }       ?>>PHP 5</option>     </select><br />     <input type="submit" name="Submit" />   </form> <?php   } ?> 

With multiple lists, a bit more work is required, as shown in the following code. If the form has been submitted, the array of selected list elements is searched. If one element that is nonempty is found, the process is complete and the user can be congratulated for the successful completion of the form. Otherwise, the form is displayed again.

Validating Multiple Lists (mandatory-list-multiple.php; excerpt)

 <?php   $ok = false;   if (isset($_POST['Submit']) &&       isset($_POST['multilistname'])) {     if (is_array($_POST['multilistname'])) {       for ($i=0; $i <         count($_POST['multilistname']); $i++) {         if ($_POST['multilistname'][$i] != '') {           $ok = true;           break;         }       }     }   }   if ($ok) {     echo '<h1>Thank you for filling out this       form!</h1>';   } else { ?>   <form method="post" action=" <?php echo htmlspecialchars($_SERVER['PHP_SELF']);    ?>">     <select name="multilistname[]"       multiple="multiple" size="5">     </select> <br />     <input type="submit" name="Submit" />   </form> <?php   } ?>