为什么这是无限递归？

Just ask the title asks, here's the code:

<?php

$foo = 0;

function letsLoop() {
    while ($foo != -1) {
        changeFoo();
        echo "Hello, world!
";
    }
}

function changeFoo() {
    extract($GLOBALS);

    $foo = -1;
}

letsLoop();

Just because you extract the global into the local scope doesn't mean that you will be modifying the global variable $foo. To reference the global variable inside the function, you must use the global keyword:

function changeFoo() {
    global $foo;

    $foo = -1;
}