<form method='post'>
<?php
$file = file('list.txt');
if(!empty($_POST))
{
   file_put_contents('list.txt', '');
   foreach($file as $key => $value) {
        if(!isset($_POST['s'.$key])) {
            //unset($_POST['s'],$key);
            $mol = [$key => $value];
            foreach($mol as $v) {
                file_put_contents('list.txt', $v, FILE_APPEND);
            }
        }
    }
    $file = file('list.txt');
    foreach($file as $key => $value) {
        echo "<input type='checkbox' name='s".$key."' value={$key}>{$value}<br/>";
    }
} else {
    foreach($file as $key => $value) {
        echo "<input type='checkbox' name='s".$key."' value={$key}>{$value}<br/>";
    }
}
?>
<input type='submit' value='send'/>
</form>