How Static Variables Work with Inheritance in PHP

results.txt

On PHP 7.2.24-0ubuntu0.18.04.1 (cli) (built: Oct 28 2019 12:07:07) ( NTS ):

ClassA = 1576680836
ClassA = 1576680836
ClassA = 1576680838
ClassA = 1576680838

ClassA = 1576680840
ClassA = 1576680840
ClassX = 1576680842
ClassX = 1576680842

ClassA = 1576680844
ClassA = 1576680844
ClassY = 1576680846
ClassY = 1576680846

test.php

<?php
class ClassA {
  function test1() {
    static $x = NULL;

    if (empty($x)) {
      $x = time();
    }

    print __CLASS__ . " = " . $x . "\n";
  }

  function test2() {
    static $x = NULL;

    if (empty($x)) {
      $x = time();
    }

    print __CLASS__ . " = " . $x . "\n";
  }
}

class ClassX extends ClassA {
  function test2() {
    static $x = NULL;

    if (empty($x)) {
      $x = time();
    }

    print __CLASS__ . " = " . $x . "\n";
  }
}

class ClassY extends ClassA {
  function test2() {
    static $x = NULL;

    if (empty($x)) {
      $x = time();
    }

    print __CLASS__ . " = " . $x . "\n";
  }
}


(new ClassA())->test1();
sleep(1);
(new ClassA())->test1();
sleep(1);
(new ClassA())->test2();
sleep(1);
(new ClassA())->test2();
sleep(1);

print "\n";

(new ClassX())->test1();
sleep(1);
(new ClassX())->test1();
sleep(1);
(new ClassX())->test2();
sleep(1);
(new ClassX())->test2();
sleep(1);

print "\n";

(new ClassY())->test1();
sleep(1);
(new ClassY())->test1();
sleep(1);
(new ClassY())->test2();
sleep(1);
(new ClassY())->test2();
sleep(1);

print "\n";
print "\n";