Saving the Selected Value in a Checkbox After Submission in PHP

Let's now make it so that the value of the checkbox is saved after submission. For this, we will check that $_GET['flag'] exists (meaning the form was submitted) and is equal to one (meaning the checkbox is checked).

If these two conditions are met, then we output the checked attribute in the checkbox:

html<form action="" method="GET">
  <input type="hidden" name="flag" value="0">
  <input
    type="checkbox"
    name="flag" <?php
      if (isset($_GET['flag']) and $_GET['flag'] === '1')
        echo 'checked';
      ?>
    >
  <input type="submit">
</form>

The check can be simplified if we know for sure that the hidden input sends 0. In this case, if the checkbox is not checked, then $_GET['flag'] will contain '0', and if the form has not been submitted yet, it will contain null.

In both of these cases, we should not output checked. And we can catch both of these cases with the empty function. Thus, we can check that $_GET['flag'] is not empty, and only in this case output checked:

html<form action="" method="GET">
  <input type="hidden" name="flag" value="0">
  <input
    type="checkbox"
    name="flag"
    value="1"
    <?php if (!empty($_GET['flag'])) echo 'checked' ?>
  >
  <input type="submit">
</form>