Битовая маска

function working_days(){ 
    $days = 7;
    $working_days = 0;
    $formValue = array($_POST['mon'], $_POST['tue'], $_POST['wed'], $_POST['thu'], $_POST['fri'], $_POST['sat'], $_POST['sun']);
    for($i = 0; $i < $days; $i++){
        if($formValue != 0){
            $working_days |= $formValue[$i] << $i;
        }
    }
        
    return($working_days);
}

$working_days = 0x4b; // -s-- t-tm
$a = array();
$i = 0x1;
while ($i < 0x80) {
    $a[] = (int)(bool)($working_days & $i);
    $i <<= 1;
}
print_r($a);

$working_days = 0x4b; // -s-- t-tm
$a = str_split(strrev(substr(str_pad(decbin($working_days), '0', 7), 0, 7)), 1);
print_r($a);

function working_days_reverse($working_days){ 
    $days = 7;
    $formValue = array();
    for($i = 0; $i < $days; $i++){
        $formValue[$i] = (pow(2,$i) & $working_days) != 0 ? 1 : 0;
    }
        
    return($formValue);
}