I needed a file uploader for a client a little while ago, then the client didn't want it, so I'll share with all of you. I know I hated coding it, it was confusing (for me anyway), but I made it fairly simple to use:

<?
function uploader($num_of_uploads=1, $file_types_array=array("txt"), $max_file_size=1048576, $upload_dir=""){
  if(!is_numeric($max_file_size)){
   $max_file_size = 1048576;
  }
  if(!isset($_POST["submitted"])){
   $form = "<form action='".$PHP_SELF."' method='post' enctype='multipart/form-data'>Upload files:<br /><input type='hidden' name='submitted' value='TRUE' id='".time()."'><input type='hidden' name='MAX_FILE_SIZE' value='".$max_file_size."'>";
   for($x=0;$x<$num_of_uploads;$x++){
     $form .= "<input type='file' name='file[]'><font color='red'>*</font><br />";
   }
   $form .= "<input type='submit' value='Upload'><br /><font color='red'>*</font>Maximum file length (minus extension) is 15 characters. Anything over that will be cut to only 15 characters. Valid file type(s): ";
   for($x=0;$x<count($file_types_array);$x++){
     if($x<count($file_types_array)-1){
       $form .= $file_types_array[$x].", ";
     }else{
       $form .= $file_types_array[$x].".";
     }
   }
   $form .= "</form>";
   echo($form);
  }else{
   foreach($_FILES["file"]["error"] as $key => $value){
     if($_FILES["file"]["name"][$key]!=""){
       if($value==UPLOAD_ERR_OK){
         $origfilename = $_FILES["file"]["name"][$key];
         $filename = explode(".", $_FILES["file"]["name"][$key]);
         $filenameext = $filename[count($filename)-1];
         unset($filename[count($filename)-1]);
         $filename = implode(".", $filename);
         $filename = substr($filename, 0, 15).".".$filenameext;
         $file_ext_allow = FALSE;
         for($x=0;$x<count($file_types_array);$x++){
           if($filenameext==$file_types_array[$x]){
             $file_ext_allow = TRUE;
           }
         }
         if($file_ext_allow){
           if($_FILES["file"]["size"][$key]<$max_file_size){
             if(move_uploaded_file($_FILES["file"]["tmp_name"][$key], $upload_dir.$filename)){
               echo("File uploaded successfully. - <a href='".$upload_dir.$filename."' target='_blank'>".$filename."</a><br />");
             }else{
               echo($origfilename." was not successfully uploaded<br />");
             }
           }else{
             echo($origfilename." was too big, not uploaded<br />");
           }
         }else{
           echo($origfilename." had an invalid file extension, not uploaded<br />");
         }
       }else{
         echo($origfilename." was not successfully uploaded<br />");
       }
     }
   }
  }
}
?>

uploader([int num_uploads [, arr file_types [, int file_size [, str upload_dir ]]]]);

num_uploads = Number of uploads to handle at once.

file_types = An array of all the file types you wish to use. The default is txt only.

file_size = The maximum file size of EACH file. A non-number will results in using the default 1mb filesize.

upload_dir = The directory to upload to, make sure this ends with a /

This functions echo()'s the whole uploader, and submits to itself, you need not do a thing but put uploader(); to have a simple 1 file upload with all defaults.