PHP filter_var函数输出

Question

I want to understand PHP filter_var function. I have the following code, but it does not print something in the screen. Why? Should't it print 'Hi'?

$string = "Hi";
$ad =filter_var($string, FILTER_VALIDATE_REGEXP, array("options"=>array("regexp"=>"/^T(.*)/"))); //begins with T

if ($ad === false)
    echo hi;