When I try

foreach ([true, false, null] as $i)
foreach ([true, false, null] as $j) {
  $v = variant_imp($i, $j);
  $v = (variant_get_type($v) == VT_NULL ? null : (int)$v);
  printf("variant_imp(%s, %s) = %s\n", json_encode($i), json_encode($j), json_encode($v));
}

I get

variant_imp(true, true) = -1
variant_imp(true, false) = 0
variant_imp(true, null) = null
variant_imp(false, true) = -1
variant_imp(false, false) = -1
variant_imp(false, null) = -1
variant_imp(null, true) = -1
variant_imp(null, false) = null
variant_imp(null, null) = null

true->false should be false ("true implies false" is wrong). true->null, null->false, and null->null should all be unknown.

The problem is more complex. It is bitwise operation.

$T = new VARIANT(255,VT_UI1);
$F = new VARIANT(0,VT_UI1);
$N = new VARIANT(VT_NULL,VT_UI1);

echo variant_imp($T, $F).PHP_EOL;
echo variant_imp($T, $T).PHP_EOL;
echo variant_imp($T, $N).PHP_EOL;
echo variant_imp($F, $F).PHP_EOL;
echo variant_imp($F, $T).PHP_EOL;
echo variant_imp($F, $N).PHP_EOL;
echo variant_imp($N, $F).PHP_EOL;
echo variant_imp($N, $T).PHP_EOL;
echo variant_imp($N, $N).PHP_EOL;

0
255
1  <--- Not null
255
255
255
254 <--- Not null. True in all bits except one
255 
255 <--- Totaly not null. True in all bits

My mistake in previsious comment.
You are right,
"true->false should be false ("true implies false" is wrong). true->null, null->false, and null->null should all be unknown."