话不多说，一段代码实验下

<?php
class A{
    public function a1(){
        echo __FUNCTION__;
        echo "\n";
        echo __METHOD__;
         echo "\n\n";
    }

    public static function a2(){
        echo __FUNCTION__;
        echo "\n";
        echo __METHOD__;
        echo "\n\n";
    }
}

function a3(){
    echo __FUNCTION__;
    echo "\n";
    echo __METHOD__;
    echo "\n\n";
}

$a = new A();
$a->a1();
A::a2();
a3();

输出如下

a1
A::a1

a2
A::a2

a3
a3

结论

• 对于类成员函数，METHOD会带上类名，FUNCTION 没有类名
• 对于非类成员函数，没差别