PHP Resize Image - Keep Dimensions

<?php
error_reporting(0);

$image_size = stripslashes($_GET['s']);
$image_source = stripslashes($_GET['f']);
$image_extension = stripslashes($_GET['x']);
  if ($image_extension == 'png') {
    $src = imagecreatefrompng($image_source);
  }
  if ($image_extension == 'jpg') {
    $src = imagecreatefromjpeg($image_source);
  }
  if ($image_extension == 'jpeg') {
    $src = imagecreatefromjpeg($image_source);
  }
$dw = $image_size;
$dh = $image_size;
$sw = imagesx($src);
$sh = imagesy($src);
$wr = $sw / $dw;
$hr = $sh / $dh;
$cx = 0;
$cy = 0;
if ($hr < $wr)
{
    $ow = $sw;
    $sw = $dw * $hr;
    $cx = ($ow - $sw) / 2;
}
if ($wr < $hr)
{
    $oh = $sh;
    $sh = $dh * $wr;
    $cy = ($oh - $sh) / 2;
}
$dst = imagecreatetruecolor($dw, $dh);
imagecopyresampled($dst, $src, 0, 0, $cx, $cy, $dw, $dh, $sw, $sh);
header('Content-type: image/png');
imagepng($dst);
?>