You can use echo to print something: docs
$_POST is an array, if you want to examine its value for debug purposes use var_dump (docs):
var_dump($_POST);
To access array's elements, use square brackets: $_POST['index']. PHP arrays may be associative, which means they are indexed not only by numbers, but also by strings. $_POST is a pure associative array, indexed only with strings.
To print POST parameter called username use this code:
echo $_POST['username'];
or
echo($_POST['username']);
Both will work because echo is not a function, but a language construct (see docs linked above).
echo $post...