Error in upload file and xml parsing

There is an at uploading xml file and in parsing.However, I don't have any experience with handling uploaded files in PHP yet. So thanx in advance.Should I use the $_FILES and do I always have to use a temporary file or can it be done completely in memory? code:

switch($cmdButton){
    case "parse an xml document";
    $default_dir = "";
    $xml_string = file_get_contents($default_dir . "/" . "file");
    $xp=xml_parser_create();
    xml_parser_into_struct($xp, $xml_string, $values, $keys);
    xml_parser_free($xp);

    break;
}
}
?>
<table width="100%" border="1">
<tr><td><font face="Arial, Helvatica, sans-serif" size="-1">
<b>parse an xml document</b></font></td></tr><tr><td>
<font face="Arial, Helvatica, sans-serif" size="-1">
<b>choose xml document</b></font></td></tr>
<td>
<?php
echo "<select name=\"file\">\n";
foreach (new DirectoryIterator('.') as $file) {
   // if the file is not this file, and does not start with a '.' or '..',
   // then store it for later display
   if ( (!$file->isDot()) && ($file->getFilename() != basename($_SERVER['PHP_SELF'])) ) {
      echo "<option>";
      // if the element is a directory add to the file name "(Dir)"
      echo ($file->isDir()) ? "(Dir) ".$file->getFilename() : $file->getFilename();
      echo "</option>\n";
   }
}
echo "</select>\n";
?>
</select></td><tr><tr>
<td><font face="Arial, Helvatica, sans-serif" size="-1">
    <b>xml file contents</b></font><hr>
    <?php
    if($cmdButton =="parse an xml document"){
        echo "keys array<BR><BR><PRE>";
        print_r($keys);
        echo "</PRE><BR><BR>values array<BR><BR><PRE>";
        print_r($values);
        echo"</PRE>";
    }
    ?>