Thread ID into collapseobj

[TygerTyger]

PHP syntax isn't my strongest point...I want to put a $thread[threadid] value inside a $vbcollapse[collapseobj_threadidgoeshere] and it's giving me:

Parse error: parse error, unexpected T_VARIABLE, expecting ']'

The $thread[threadid] is used inside tlist inside the same template, what is the problem with putting it where I want?

Thanks for your help.

Edit: Specifically this is what I want:

id="collapseobj_foo$thread[threadid]" style="$vbcollapse[collapseobj_foo$thread[threadid]]"

The id is fine, the style one gives the error. As yet curly braces and/or escaping have made no difference.