Preselecting Radio Buttons

Radio buttons come in groups identified by using the same name attribute on all buttons within that group. A user can only select one radio button from a group of radio buttons. When a radio button is selected, any other radio button with the same name (in the group) that was previously selected becomes unchecked.

The checked attribute

Use the checked attribute to pre-select a radio button, for example:

checked
or
checked=""
or
checked="checked"

Each of these is a valid technique to define a checked radio button.

Example: pre-checked a radio button:

The last radio button (Blue) will be checked:

<input type="radio" name="color" value="Red"> Red
<input type="radio" name="color" value="Green"> Green
<input checked type="radio" name="color" value="Blue"> Blue

PHP – Preselecting Radio Buttons

<?php
 if (isset($_POST['groupname']) &&
  $_POST['groupname'] == 'any value') {
   echo 'checked';
 }

When processing a form in PHP, it is quite messy but rather trivial to prefill a group of radio buttons: Just compare the value in $_GET/$_POST with the associated value. If it fits, print out checked to preselect that radio button, as shown in the code:

<?php
 $color = htmlspecialchars ($_POST['color'] ?? ''); 
?>
<form method="post" action="example.php">
 <p>Radio Buttons:<br>
  <label>
<input type="radio" name="color" value="Red"
 <?= $color == 'Red' ? 'checked' : '' ?>>Red
  </label>
  <label>
<input type="radio" name="color" value="Blue"
 <?= $color == 'Blue' ? 'checked' : '' ?>>Blue
  </label>
  <label>
<input type="radio" name="color" value="Green"
 <?= $color == 'Green' ? 'checked' : '' ?>>Green
  </label>
 </p>
 <p><input type="submit" name="submit" value="Submit"></p>
</form>

Using a cookie to check the radio button

This code can be extended so that a radio button is preselected when the user has previously saved his selection in a cookie:

<?php
 if (isset($_POST['submit'])) {
  $color = htmlspecialchars ($_POST['color'] ?? '');
  
  // Save the form data in cookies when form is submitted
  setcookie('color', $color, strtotime ('+30 days'));
 }
 else {
  // Form not submitted
  // Retrieve data from cookies (if exist)
  $color = $_COOKIE['color'] ?? '';
 }
 
?>
<form method="post" action="example.php">
 <p>Radio Buttons:<br>
  <label>
   <input type="radio" name="color" value="Red"
    <?= $color == 'Red' ? 'checked' : '' ?>>Red
  </label>
  <label>
   <input type="radio" name="color" value="Blue"
    <?= $color == 'Blue' ? 'checked' : '' ?>>Blue
  </label>
  <label>
   <input type="radio" name="color" value="Green"
    <?= $color == 'Green' ? 'checked' : '' ?>>Green
  </label>
 </p>
 <p><input type="submit" name="submit" value="Submit"></p>
</form>

---

Processing Forms in PHP: