Friday afternoon problem array question

[eluser]tim1965[/eluser]
Sorry in advance i know this is easy. But my brain doesnt seem to functioning today.
I want to add
$propid = $this->session->userdata('propid');
to a POST array
If i var_dump $propid i get

array(1) { ["prop_id"]=> array(1) { [0]=> string(1) "5" } }

So if i use
$form['property_id'] = $propid['prop_id'];
It adds "array" rather than "5", which is the value i am trying to get

Any help appreciated