无法将变量从PHP函数传递到同一文件中的另一个函数

function function1() {
    global $variable;
    $variable = "Hello";
}

function function2() {
    global $variable;
    echo $variable; // 输出 "Hello"
}

function1();
function2();

function function1() {
    static $variable = "Hello";
    return $variable;
}

function function2() {
    $variable = function1();
    echo $variable; // 输出 "Hello"
}

function2();

function function1($variable) {
    $variable = "Hello";
    return $variable;
}

function function2($variable) {
    echo $variable; // 输出 "Hello"
}

$variable = "";
$variable = function1($variable);
function2($variable);