[PHP] ImageCopyResized() function

[BBC]

Hi all.
is any one know how to use these functions, and what for are they:
imagecreatetruecolor();
imagecreatefromjpeg();
ImageCopyResized();
ImageDestroy();

please tell me how to use it like this function
mail($a,$b,$c,$d);
$a = "email direction where we email to";
$b = "subject of email, it will show on the inbox as a title";
$c = "the content of email";
$d = "type 'From :' and then followed by email where we send from";