Como redimensionar imagens com PHP

php
function redimensiona($filename, $tmpname, $xmax, $ymax)
{
    $ext = explode(".", $filename);
 
    $ext = $ext[count($ext)-1];
 
    if($ext == "jpg" || $ext == "jpeg")
        $im = imagecreatefromjpeg($tmpname);
 
    elseif($ext == "png")
        $im = imagecreatefrompng($tmpname);
 
    elseif($ext == "gif")
        $im = imagecreatefromgif($tmpname);
 
    $x = imagesx($im);
 
    $y = imagesy($im);
 
    if($x <= $xmax && $y <= $ymax)
        return $im;
 
    if($x >= $y) {
        $newx = $xmax;
 
        $newy = $newx * $y / $x;
 
    }
    else {
        $newy = $ymax;
 
        $newx = $x / $y * $newy;
 
    }
    $im2 = imagecreatetruecolor($newx, $newy);
 
    imagecopyresized($im2, $im, 0, 0, 0, 0, floor($newx), floor($newy), $x, $y);
 
    return $im2;
 
}