JuniorTheoryCommon

What happens when you run this program?

PHP

<?php

$rate = 2;

function apply($x)
{
    return $x * $rate;
}

echo apply(10), "\n";

A Warning: Undefined variable $rate, then 0

Expecting 20, as in JavaScript: a PHP function does not see the variables of the file around it, so $rate has to come in as a parameter.
Expecting an undefined variable to stop the program: it is only a Warning, PHP uses null in its place, and 10 * null gives 0.

JuniorTheoryCommon

PHP

<?php

$rate = 2;

function apply($x)
{
    return $x * $rate;
}

echo apply(10), "\n";

A Warning: Undefined variable $rate, then 0

Expecting 20, as in JavaScript: a PHP function does not see the variables of the file around it, so $rate has to come in as a parameter.
Expecting an undefined variable to stop the program: it is only a Warning, PHP uses null in its place, and 10 * null gives 0.