If you test an element of an array (like $_POST['key]), it test if the key doesn't exist or if it exist if its value is empty and never emit a warning.

For who don't want to test what happen if passed an array element here is my test and result.

<?php
$a = array();
$b = array('key');
$c = array('key' => false);
$d = array('key' => 'La verdad nos hace libres');

echo (empty($a['key'])?'A empty':'A not empty'), ' - ', (empty($b['key'])?'B empty':'B not empty'), ' - ', (empty($c['key'])?'C empty':'C not empty'), ' - ', (empty($d['key'])?'D empty':'D not empty');
?>

And the result is:

    A empty - B empty - C empty - D not empty