resize billede

hej jeg har følgende kode til at resize et billede.

hvordan får jeg den til at resize den orginale billede til et bestem width???

function createT_img($imgName, $sou, $mappen) {

    //resize image to put it into db
    $imagename = $imgName;
    $source = $sou; //$_FILES['img1']['tmp_name'];
    $target = $mappen."/".$imagename;
    $map = $mappen;
    move_uploaded_file($source, $target);
   
    $imagepath = $imagename;
    $save = $mappen ."/sml/". $imagepath; //This is the new file you saving
    $file = $mappen ."/". $imagepath; //This is the original file
   
    list($width, $height) = getimagesize($file) ;
   
    $modwidth = $width;
   
    $diff = $width / $modwidth;
   
    $modheight = $height / $diff;
    $tn = imagecreatetruecolor($modwidth, $modheight) ;
    $image = imagecreatefromjpeg($file) ;
    imagecopyresampled($tn, $image, 0, 0, 0, 0, $modwidth, $modheight, $width, $height) ;
   
    imagejpeg($tn, $save, 100) ;
   
    $save =  $mappen ."/sml/" . $imagepath; //This is the new file you saving
    $file =  $mappen ."/". $imagepath; //This is the original file
   
    list($width, $height) = getimagesize($file) ;
   
    $modwidth = 180;
   
    $diff = $width / $modwidth;
   
    $modheight = $height / $diff;
    $tn = imagecreatetruecolor($modwidth, $modheight) ;
    $image = imagecreatefromjpeg($file) ;
    imagecopyresampled($tn, $image, 0, 0, 0, 0, $modwidth, $modheight, $width, $height) ;
   
    imagejpeg($tn, $save, 100) ;
    return $imagepath;
   
   
    }