Convert JPG/GIF image to PNG in PHP?

Based on what kind of image it is you could select the correct function to open the file:

$extension = strtolower(pathinfo($filename, PATHINFO_EXTENSION)); 
switch ($extension) {
    case 'jpg':
    case 'jpeg':
       $image = imagecreatefromjpeg($filename);
    break;
    case 'gif':
       $image = imagecreatefromgif($filename);
    break;
    case 'png':
       $image = imagecreatefrompng($filename);
    break;
}

Then you just save the file using:

imagepng($image, $new_filename, $quality);

It might be worth noting that this will only parse the actual extension and not really validate so the file is in the specific format. For instance, you could take a jpg image and just change the extension to png. So a better approach is to use the exif_imagetype() which will return the type of image not depending on the actual extension.

You just need imagepng() then. In fact it almost becomes a one-liner:

 imagepng(imagecreatefromstring(file_get_contents($filename)), "output.png");

You would use $_FILES["id"]["tmp_name"] for the filename, and a different output filename obviously. But the image format probing itself would become redundant.