[PHP] Images problem

[Phpu]

If i have 2 images:

$im1 = imagecreate (100, 100); 
$white = ImageColorAllocate ($im1, 255, 255, 255);


$im2 = imagecreate (50, 50); 
$black = ImageColorAllocate ($im2, 0, 0, 0);


How can i put $im2 over $im1 and result one single image? Is there a function 
to do this?

Thank You