Users often forget to (or don't want to) fill in certain fields in a form.
When this happens, some data is not sent to the server.
Sometimes the field is sent as an empty string; sometimes no field name is sent at all.
The following table illustrates the behavior of various form controls when they're not filled in or selected:
| Form Control | What Happens When It's Not Filled In Or Selected |
|---|---|
| Text input field | The field name is sent, along with an empty value. |
| Password field | The field name is sent, along with an empty value. |
| Checkbox field | Nothing is sent at all. |
| Radio button field | Nothing is sent at all. |
| Submit button | Nothing is sent at all if the button isn't clicked. This can happen if the user presses Enter/Return to submit a form. |
| Reset button | Nothing is ever sent. |
| File select field | The field name is sent, along with an empty value. |
| Hidden field | The field name is sent, along with an empty value. |
| Image field | Same behavior as a submit button. |
| Push button | Nothing is ever sent. |
| Pull-down menu | Impossible to select no option, so a value is always sent. |
| List box | Nothing is sent at all. |
| Multi-select box | Nothing is sent at all. |
| Text area field | The field name is sent, along with an empty value. |
When nothing is sent at all for a field, PHP doesn't create an element for the field in the $_POST, $_GET, or $_REQUEST array.
So if you attempt to access the element, you'll generate a PHP notice along the lines of:
You can check if a field has value using PHP functions such as isset() or array_key_exists():
<dt>Gender</dt><dd><?php if (isset($_POST["gender"])) echo $_POST["gender"]?></dd>