函数运行后函数中的变量是否会被销毁

函数运行后函数中的变量会被销毁吗？

class B {
   function C() {
       $x = "123456";
       echo "XXX".$x;
       // After this function is finished, will $x be destroyed by default to save memory in PHP?
   }
}
class A {
   function F1() {
       return new Class_B();
   }
   function F2() {
       $this->F1()->C();
       // After this function is finished, will F1 be destroyed by default to save memory and CPU in PHP?
   }
}