Q:

how to use php $_POST[something] not user ' after [?

my code 
<?php
    if (isset($_POST)) {
        echo $_POST['username'];
    }
?>

but i want use $_POST[username] not use string ' .How to can i do it?

A:

You cannot use without single quote in POST
You can declare $postName = 'username'; and use this variable.
<?php
$postName = 'username';

if (isset($_POST)) {
    echo $_POST[$postName];
}
?>

