a.php

$a = function () {
  echo 'x';
};

$a();

b-broken.php

$x = 4;

$a = function () {
  echo $x; // x is not defined
};

$a();

b-fixed.php

$x = 4;

$b = function () use ($x) {
  echo $x;
};

$b();

c-broken.php

$a = new stdClass();

$a->b = function () {
  echo 'x';
};

$a->b(); // no such method 'b'

c-fixed.php

$a = new stdClass();

$a->b = function () {
  echo 'x';
};

call_user_func($a->b); // no such method 'b'

d-broken.php

class A {
  public function b() {
    return function () {
      $this->c(); // this cannot be used inside a closure
    };
  }

  public function c() {
    
  };
}

$a = new A();
$a->b();

d-fixed-kinda.php

class A {
  public function b() {
    $self = $this;
    return function () use ($self) {
      $self->c();
      $self->d(); // call to a protected class from outside the object
    };
  }

  public function c() {
    
  };

  protected function d() {
    
  };
}

$a = new A();
$a->b();