如何调用受保护的功能？

How to call function lmn() without touching class B from class A

 class A extends B{
     public function abc(){
         return "abc";
     }
     ...
 }

 class B{
     public function xyz(){
         return "xyz";
     }
     ...
 }

 class C{
     protected function lmn(){
         return "lmn";
     }
     ...
 }

please guide me for this

You cannot call it since protected means to be a function that can be called from the child classes.

In your case, you need to make an instance of B to call lmn in whatever class.

You can make some kind of proxy class that extends from C and provides a public method for access:

class ProcyForC extends C {
  public function getLmn() {
    return $this->lmn();
  }
}

echo (new ProxyForC())->getLmn();