PHP difference between saving .gif and .png files to disk - php

Question

I'm keen to understand the difference between saving a .gif or .jpg (which seem to work fine using the code below), but does not work if I change it to .png
So this code works fine for .jpg or .gif images (links ending in .jpg or .gif)
$img = "http://www.anylocation.com/image.gif";
$ch = curl_init($img);
$fp = fopen($currentdir . 'image.gif', 'wb');
curl_setopt($ch, CURLOPT_FILE, $fp);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_exec($ch);
curl_close($ch);
fclose($fp);
echo "<b>Image stored...<br>";
But if I do this:
$img = "http://www.anylocation.com/image.png";
$ch = curl_init($img);
$fp = fopen($currentdir . 'image.png', 'wb');
curl_setopt($ch, CURLOPT_FILE, $fp);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_exec($ch);
curl_close($ch);
fclose($fp);
echo "<b>Image stored...<br>";
It doesn't save the image, well, it tries to, but it just writes image.png with 0 byte size and no content.
Ideally I'd like to modify the above code so that it will work for any scenario or type of image. If not, and the code has to be different for .png, I could write in a 'check the extension of $img' and direct it to the correct piece of code.
Could anyone assist with the explanation of why this doesn't work with .png and if possible, any changes that I could make to make it work?

Answer 1

Your code can reduce to ...
$img = "http://www.anylocation.com/image.png";
file_put_contents($currentdir . 'image.png', file_get_contents($img));