So you have an array with the $_POST
response, let's say $zutaten=$_POST['zutaten']
for convenience.
Do the following for all 4 entries. You can do it dynamically if you have the possible values in an array, let me know if you need help with that.
<input type="checkbox" <?php if (in_array('b', $zutaten)) echo 'checked="checked"'; ?> id="zutaten" name="zutaten[]" value="b" >
EDIT: Don't forget to keep in $zutaten
only the values that were checked. If you have an array. You can do the following:
$values = array('a', 'b', 'c', 'd');
foreach ($values as $val) {
<input name="zutaten[]" value="<?php echo $val; ?>" type="checkbox" <?php if (in_array($val, $zutaten)) echo 'checked="checked"'; ?>
}