Fatal error: Array callback has to contain indices 0 and 1

PHP Snippet 1:

$asin = $_POST('asin');

PHP Snippet 2:

$asin = $_POST['asin'];

PHP Snippet 3:

<?php
    class A {
        static function ech($a) {echo $a;}
        function echa($a) {echo $a;}
    }
    ['A', 'ech']('Test');
    [new A, 'echa']('Test');