if both "if_available" and "required" on AX request are specified, not all return values are in response.

code overwrites $short_names array rather than merging it.

I've changed the code as follows:

if (isset($attributes['openid.ax.if_available'])) {
$short_names += explode(',', $attributes['openid.ax.if_available']);
}
if (isset($attributes['openid.ax.required'])) {
// $short_names += explode(',', $attributes['openid.ax.required']);
$short_names = array_merge($short_names, explode(',', $attributes['openid.ax.required']));
}

by merging the array, we're no longer overwriting the initial entries, but adding to it.