create thumbnail from images in PHP

Question

if(preg_match('/[.](jpg)$/', $filename)) {

        $im = imagecreatefromjpeg($file . $filename);

    } else if (preg_match('/[.](gif)$/', $filename)) {

        $im = imagecreatefromgif($file . $filename);

    } else if (preg_match('/[.](png)$/', $filename)) {

        $im = imagecreatefrompng($file . $filename);

    }

 

    $ox = imagesx($im);

    $oy = imagesy($im);

     

    $nx = $final_width_of_image;

    $ny = floor($oy * ($final_width_of_image / $ox));

     

    $nm = imagecreatetruecolor($nx, $ny);

  

    imagecopyresized($nm, $im, 0,0,0,0,$nx,$ny,$ox,$oy);

    

    imagejpeg($nm, $thumnailDir . $filename,100);

    $tn = '<img src="' . $thumnailDir . $filename . '" alt="image" />';

    $tn .= '<br />Congratulations. Your file has been successfully uploaded, and a thumbnail has been created.';

    echo $tn;

here is the code m using for creating a thumbnail image... but it returns me an image of 0 bytes... can any one help me with this where I am making mistake?