What is the PHP syntax to check “is not null” or an empty string?

Null OR an empty string?

if (!empty($user)) {}

Use empty().

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After realizing that $user ~= $_POST[‘user’] (thanks matt):

var uservariable='<?php 
    echo ((array_key_exists('user',$_POST)) || (!empty($_POST['user']))) ? $_POST['user'] : 'Empty Username Input';
?>';