Validating data = Determine if the data is in proper form.

Sanitizing data = Remove any illegal character from the data.

PHP filters are used to validate and sanitize external input.

The PHP filter extension has many of the functions needed for checking user input, and is designed to make data validation easier and quicker.

The filter_list() function can be used to list what the PHP filter extension offers:

<table>
  <tr>
    <td>Filter Name</td>
    <td>Filter ID</td>
  </tr>
  <?php
  foreach (filter_list() as $id =>$filter) {
    echo '<tr><td>' . $filter . '</td><td>' . filter_id($filter) . '</td></tr>';
  }
  ?>
</table>