<?php

class MyClass
{
    // find out where $myClass->exec() was called
    static function getInstantiatingClass()
    {   
        return debug_backtrace(2, 3)[2]['class'];
    }

    function exec()
    {
        echo __METHOD__ . ' was called from Class ' . self::getInstantiatingClass();
    }
}

class Demo
{
    function test()
    {
        $myClass = new MyClass;
        $myClass->exec();
    }
}

$demo = new Demo;
$demo->test();