Does PHP negation check with `!` coprrespond to `!=` or to `!==`?

Question

In PHP, is

if(!$foo)

equivalent with

if($foo != true)

or with

if($foo !== true)

or is it even something completly different of both?

Answer 1

if(!$foo)

is the equivalent to

if($foo != true)

so

$foo = null;
if(!$foo){
 echo "asd";
}

will ouptut "asd"