Resize image php

The following code resizes a png image and displays it as a jpg image:

<?php
    header("content-type: image/jpg");
    $file_name = 'img.png';
   
    $srcsize = getimagesize($file_name);
   
    $ext = strrchr($file_name,'.');
    $ext = strtolower($ext);
         
    if($ext == ".jpg"){
               $new_img_big = imagecreatefromjpeg($file_name);
    }elseif($ext == ".png"){
               $new_img_big = imagecreatefrompng($file_name);
    }elseif($ext == ".gif"){
               $new_img_big = imagecreatefromgif($file_name);
    }
   
    $resized_img_big = imagecreatetruecolor(100, 100);
    imagecopyresized($resized_img_big, $new_img_big, 0, 0, 0, 0, 100, 100, $srcsize[0], $srcsize[1]);
        
    imagejpeg($resized_img_big);         
   
    /*imagedestroy($src_img);
    imagedestroy($dst_img);*/
?>