Bitwise operations criticism needed

The situation. Im currently using a home brewed groups permission code in my site, but for limited users/groups its ok. Beyond that, the code will take the fast road to hell. I started to look in depth at bitwise operations today, and after much googling, and looking at other code, came up with this mock up, of a basic permissions routine (not neccessarilly the final product), that utilizes bitwise operations. Im looking for advise, to see if Im heading in the right way, and/or improvements with this. Thanks for your time... -- start code -- <?php $perm = $user = array(); $perm_total = 0; // Permissions from some source like a database/file $perm['execute'] = 1; $perm['write'] = 2;

$perm['read']    = 4;

// User permissions (predetermined) from sessions maybe

$user['tom']            = 7;  // rwx
$user['joe']            = 5;  // rx
$user['dirty_harry']    = 9;  // illegal in this case??

$user['whats_his_face'] = 6; // rw // Set the sum of "source" permissions foreach($perm as $value) {

    $perm_total |= $value;

} echo "<ul>"; // Loop over the users foreach($user as $id => $user_perm) {

    // User permissions should be between 1 & 7, else set it to 0
    if ($user_perm > $perm_total || $user_perm < 0)
    {
        $user_perm = 0;
    }

    // Compare user bits to permission bits
    $compare = $perm_total & $user_perm;

    // Make it an even 4 bit string (for visual effect)
    $bits_string = sprintf("%03d", decbin( $compare ));

    echo "<li>User: " . $id . "</li>";

    // Check to see if the comparision contains any permission bits
    $can_read    = (($compare & $perm['read']) == $perm['read']) === TRUE ? "TRUE" : "FALSE";
    $can_write   = (($compare & $perm['write']) == $perm['write']) === TRUE ? "TRUE" : "FALSE";
    $can_execute = (($compare & $perm['execute']) == $perm['execute']) === TRUE ? "TRUE" : "FALSE";

    echo "<ul>";
    echo "<li>" . $user_perm . ' -> ' . $bits_string . "</li>";
    echo "<li>Can Read: $can_read</li>";
    echo "<li>Can Write: $can_write</li>";
    echo "<li>Can Execute: $can_execute</li>";

    echo "</ul>";

} echo "</ul>"; ?> -- end code --