Try to use utf8-encode() like,
$file = utf8_encode($_POST['file_path']); // use utf8-encode here
$content = $_POST['file_content'];
//get vars from post
$path = explode("/",$file);
$path_start = utf8_encode($path[0]."/".$path[1]); // use utf8-encode here
//define path_start
$i = 1;
while ($file != $path_start) {
if (!file_exists($path_start)) {
mkdir($path_start,0777);
}//end if
$i++;
$path_start = utf8_encode($path_start."/".$path[$i]); // use utf8-encode here
} //end while
// your remaining code here