php array_search 0 index

This is explicitly mentioned in the docs. You need to use === or !==:

$key = array_search(...);

if ($key !== false) ...

Otherwise, when $key is 0, which evaluates to false when tested as a boolean.

The conditional in your second example block gives execution order priority to the !== operator, you want to do the opposite though.

if (($key = array_search(100,$test)) !== false) {

!== has higher precedence than == which makes the parentheses necessary.