how to store all checkbox values in array in php

i m trying to store checkbox values inside variable $days but when i store this value to database it just store the last selected checkbox inside the database... please help me to sort out the code so i can get all selected values

    <?php

   if (isset($_POST['days'])) {
    foreach ($_POST['days'] as $day) {
        $days = " " . $day;
    }
} else {
    $days = "not available";
}  
?>
 <form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post" enctype="multipart/form-data">
<tr>
<td>Select Days :</td>
<td>
    <input type="checkbox" name="days[]" value="Sunday">Sunday
    <input type="checkbox" name="days[]" value="Monday">Monday
    <input type="checkbox" name="days[]" value="Tuesday">Tuesday
    <input type="checkbox" name="days[]" value="Wednesday">Wednesday
    <input type="checkbox" name="days[]" value="Thursday">Thursday
    <input type="checkbox" name="days[]" value="Friday">Friday
    <input type="checkbox" name="days[]" value="Saturday">Saturday
    <input type="checkbox" name="days[]" value="All Days">All Days
</td>
<td></td>