How to scale up an image with PHP GD?

how can I do this? I h开发者_开发问答ave an image 50x50 and I would like to generate one with 100x100, where the original 50x50 will be centered inside of that one. The rest would be filled with "transparent". Thanks

---

This is how you do it:

$old = imagecreatefromjpeg("old_image.jpg"); 
// Create a 100x100 image
$im = imagecreatetruecolor(100, 100);
$black = imagecolorallocate($im, 0, 0, 0);

// Make the background transparent
imagecolortransparent($im, $black);

// Copy old image on top of new image
imagecopy($im, $old, 25, 25, 0, 0, 50, 50); 

// Save the image
imagepng($im, './new_image.png');
imagedestroy($im);