further I realized that an object, when getting detroyed, does care about destroying variable in object space visibility but not those in local visibility, be aware of the found pattern:

<?php
class release_test{
  private $buffer;
  private $other_object;
  public function __construct(){
    $this->other_object=new other_object_class();
  }
  public function __destruct(){
    //note that you always have to unset class objects, in order to get the resources released
    unset($this->other_object);
  }
  public allocate_mem_A(){
    $this->buffer=file("/tmp/bigfile");
  }
  public allocate_mem_B(){
    $buffer=file("/tmp/bigfile");
  }
  public allocate_mem_C(){
    $buffer=file("/tmp/bigfile");
    unset($buffer);
  }
  public allocate_mem_D(){
    $this->other_buffer=file("/tmp/bigfile");
  }
}

//this does not lead to a resource problem
$A=new release_test();
$A->allocate_mem_A();
$A->__destruct();
unset($A);

//this DOES lead to a resource problem
$B=new release_test();
$B->allocate_mem_B();
$B->__destruct();
unset($B);

//this does not lead to a resource problem
$C=new release_test();
$C->allocate_mem_C();
$C->__destruct();
unset($C);

//this does not lead to a resource problem
$D=new release_test();
$D->allocate_mem_D();
$D->__destruct();
unset($D);
?>