PHP mkdir() invalid argument

Question

I am engaging in a project on windows server 2016, and the XAMPP/PHP version is 5.6(it needs an update I know), and something weird just happened:
I have an index.php that collects some parameters and post them to controller.php with ajax:

function scan_setting_confirm(){
      const resp_array = [$("#year").val(),
                          $("#month").val(),
                          $("#cust_").val(),
                          $("#invoice_format").val(),
                          $("#select_save_path").val(),
                          $("#select_scan_path").val()
                          ]

      const radio_val = $("input[name='deduct']:checked").val();
      if(resp_array.indexOf("")===-1 && radio_val !== undefined){
        $.ajax({
          url:'controller.php',
          cache:false,
          dataType:'html',
          type:'POST',
          data:{
                task:'scan-setting',
                year:$("#year").val(),
                month:$("#month").val(),
                cust:$("#cust_").val(),
                invoice_format:$("#invoice_format").val(),
                select_save_path:$("#select_save_path").val(),
                select_scan_path:$("#select_scan_path").val(),
                deduct:$("input[name='deduct']:checked").val()
                },
          error:function(e){alert(e);},
          success:function(data){alert(data);}
        });
      }else{
        alert('Please check if there is anything missed.');
      }
    
    } 
    const ele3 = document.getElementById('scan_setting_btn');
    ele3.addEventListener('click',scan_setting_confirm);

and what controller.php does is:

function rename_pic(){
    $year = $_POST['year'];
    $month = $_POST['month'];
    $cust = $_POST['cust'];
    $deduct = $_POST['deduct'];
    $format = $_POST['invoice_format'];
    $scan = $_POST['select_scan_path'];
    $save = $_POST['select_save_path'];
    if($deduct=='2'){
        $format = '88';
    }
    $save_dir_father = $save.'\\'.$cust;
    $save_dir = $save.'\\'.$cust.'\\'.$year.'.'.str_repeat('0',2-strlen($month)).$month;
    if(!file_exists($save_dir_father)){
        mkdir($save_dir_father);
    }
    echo $_POST['select_scan_path'];
    mkdir($save_dir);
    echo $save_dir;
}
if ($_POST['task'] == 'scan-setting'){rename_pic()};

I tried to use those params to create directories, and this just casted an error:
But it seems pretty normal when echoing the variable,$save_dir.
I've tried encoding $_POST['select_scan_path'] and $_POST['select_save_path'] with BIG5 with iconv function, but it casted out another error:

iconv(): Detected an illegal character in input string

The weirdest thing is, if $_POST['select_scan_path'] and $_POST['select_save_path'] are encoded with big5 and the paths contains chinese characters, it works.
Codes just works fine on windows 11.
Any ideas? Thanks!

Answer 1

$save_dir_father = $save.'\\'.$cust;
$save_dir = $save.'\\'.$cust.'\\'.$year.'.'.str_repeat('0',2-strlen($month)).$month;

convert it to the following

$save_dir_father = $save.'\'.$cust;
$save_dir = $save.'\'.$cust.'\'.$year.'.'.str_repeat('0',2-strlen($month)).$month;