PHP 関数内でグローバル変数にアクセスできないのはなぜですか?

$sxml = new SimpleXMLElement('<somexml/>');

function foo(){
    $child = $sxml->addChild('child');
}

foo();

function foo($sxml){
    $child = $sxml->addChild('child');
}
foo($sxml);

function foo(){
    global $sxml;
    $child = $sxml->addChild('child');
}
foo();

function bar() {
    $sxml = new SimpleXMLElement('<somexml/>');
    $foo = function() use(&amp;$xml) {
        $child = $sxml->addChild('child');
    };
    $foo();
}
bar();

function bar() {
    $sxml = new SimpleXMLElement('<somexml/>');
    function foo() {
        $child = $sxml->addChild('child');
    }
    foo();
}
bar();