function add_file_field(){
var container=document.getElementById('file_container');
var file_field=document.createElement('input');
file_field.name='images[]';
file_field.type='file';
container.appendChild(file_field);
var br_field=document.createElement('br');
container.appendChild(br_field);
}

<form action="uploader.php" method="post" enctype="multipart/form-data" name="mutiple_file_upload_form" id="mutiple_file_upload_form">
  <h1>Advanced Multiple File Upload Script Example</h1><div id="file_container">
    <input name="images[]" type="file"  />
    <br />
  </div>
  <a href="javascript:void(0);" onClick="add_file_field();">Add another</a><br />
  <input type="submit" name="Submit" value="Submit" />
</form>

if (isset($_POST['Submit'])) {
    $number_of_file_fields = 0;
    $number_of_uploaded_files = 0;
    $number_of_moved_files = 0;
    $uploaded_files = array();
    $upload_directory = dirname(__file__) . '/uploaded/'; //set upload directory
    /**
     * we get a $_FILES['images'] array ,
     * we procee this array while iterating with simple for loop 
     * you can check this array by print_r($_FILES['images']); 
     */
    for ($i = 0; $i < count($_FILES['images']['name']); $i++) {
        $number_of_file_fields++;
        if ($_FILES['images']['name'][$i] != '') { //check if file field empty or not
            $number_of_uploaded_files++;
            $uploaded_files[] = $_FILES['images']['name'][$i];
            if (move_uploaded_file($_FILES['images']['tmp_name'][$i], $upload_directory . $_FILES['images']['name'][$i])) {
                $number_of_moved_files++;
            }
 
        }
 
    }
    echo "Number of File fields created $number_of_file_fields.<br/> ";
    echo "Number of files submitted $number_of_uploaded_files . <br/>";
    echo "Number of successfully moved files $number_of_moved_files . <br/>";
    echo "File Names are <br/>" . implode(',', $uploaded_files);
}

<script type="text/javascript">
	function validatefile(){
		var myform=document.uploadform;   //reference to form 
		var file_field_value=myform.filefield.value; //access form value with dom model
		var error="";
			if(file_field_value==''){
				error+="Please Select A file.\n";
			}
		var allowed_extensions=/(jpg|jpeg|bmp|giff|doc|docx|pdf)$/; //check for allowed extension it is simple regular expression
 
			if(!file_field_value.match(allowed_extensions)){
				error+="Please select only jpeg or jpg or bmp or giff or doc or pdf .";
			}
 
			if(error!=''){  //check for any errors
				alert(error);   //if error alert error
				return false;   //return to form don't submit to action page
			}
 
	return true;   // every thing going fine return true allow form to submit the data.
	}
 
	</script>

<?php
if (isset($_POST['Upload'])) {  //check if form submitted
    if (!empty($_FILES['filefield'])) { //check for image submitted
        if ($_FILES['filefield']['error'] > 0) { // check for error re file
            echo "Error: " . $_FILES["filefield"]["error"] . "<br />";
        } else {
            print_r($_FILES);  //every thing fine file successfully uploaded to server
        }
 
 
    } else {
        die('File not uploaded.'); // exit script
    }
}
 
?>

 if (!isset($_POST)){die('You can"t access this file directly');}//avoid direct accessing to this file.
 
 if (isset($_POST['Upload'])) {  //check if form submitted
    if (!empty($_FILES['filefield'])) { //check for image submitted
        if ($_FILES['filefield']['error'] > 0) { // check for error re file
            echo "Error: " . $_FILES["filefield"]["error"] . "<br />";
        } else {
            $file=$_FILES['filefield'];  //every thing fine. file successfully uploaded to server
        }
 
 
    } else {
        die('File not uploaded.'); // exit script
    }
}
 
 $upload_directory='uploaded/';
 $ext_str = "gif,jpg,jpeg,mp3,tiff,bmp,doc,docx,ppt,pptx,txt,pdf";
 $allowed_extensions=explode(',',$ext_str);
 $max_file_size = 10485760;//10 mb remember 1024bytes =1kbytes
 $overwrite_file = false;
 /* 
 upload directory check 
  */
 $status = true;
 if (!is_dir($upload_directory)) { //check if upload directory exists or not
            if ($mkdir) {
                if (!mkdir($upload_directory)) { //if directory doesn't exists try to create it,if fails warn user
                    $status = false;
                } else {
                    if (!chmod($upload_directory, 0777)) $status = false; //change file permisson to write,read,execute
                }
            } else {
                $status = false;
            }
} 
if(!$status){  //if can't make a directory warn the user and exit
die('There is no uploade directory or i can" create the upload directory');
}
 
/* 
check allowed extensions here
 */ 
$ext = substr($file['name'], strrpos($file['name'], '.') + 1); //get file extension from last sub string from last . character
if (!in_array($ext, $allowed_extensions) ) {
die('only'.$ext_str.' files allowed to upload'); // exit the script by warning
 
/* 
check file size of the file if it exceeds the specified size warn user
 */
 
if($file['size']>=$max_file_size){
die('only the file less than '.$max_file_size.'mb  allowed to upload'); // exit the script by warning
}
 
/* 
check if the file already exists or not in the upload directory
 */
 
if(!$overwrite_file and file_exists($upload_directory.$file['name']) ){
 die('the file  '.$file['name'].' already exists.'); // exit the script by warning
}
 
if(!move_uploaded_file($file['tmp_name'],$upload_directory.$file['name'])){
 die('The file can"t moved to target directory..'); //file can't moved with unknown reasons likr cleaning of server temperory files cleaning
}
 
 
/* 
Hurrey we uploaded a file to server successfully.
 */