array variable index in that rare SimpleXML functions

[chiqui3d]

hi very good, hi very good, I have a label on a form select and I get that value with $ _POST like this: $ _POST like this:

$ gallery = array ($ _POST ['gallery']);

and that value will put it here:

$ image = $ sitemap-> gallery [$ gallery] -> addChild ('image');

the problem is giving me error is as follows:

Fatal error: Call to a member function addChild () on a non-object in

I do not understand is that if I put a value directly asin me do it like so:

 $ gallery = 0;
$ Image = $ sitemap-> gallery [$ gallery] -> addChild ('image');

I do well, what happens is that I want the user to choose,

Kind of strange as it may fix.