self vs static vs $this

example.php

<?php

class ParentClass {

  public static string $foo = 'ooh';

  public string $bar = 'lala';

  public function __construct($arg) {
    $this->bar = $arg;
  }

  public function dump(): void {
    printf(
      "Class: %s\nself::\$foo: %s\nstatic::\$foo: %s\n\$this::\$foo: %s\n\$this->bar: %s" . PHP_EOL,
      static::class,
      self::$foo,
      static::$foo,
      $this::$foo,
      $this->bar,
    );
  }

}

class ChildClass extends ParentClass {

  public static string $foo = 'ahh';

  public string $bar = 'haha';

}

$parent = new ParentClass('parent');

$child = new ChildClass('child');

$parent->dump();
echo PHP_EOL;
$child->dump();
echo PHP_EOL;

echo "Setting ParentClass::\$foo = 'hmm'" . PHP_EOL;
ParentClass::$foo = 'hmm';

echo PHP_EOL;
$parent->dump();
echo PHP_EOL;
$child->dump();

Prints:

Class: ParentClass
self::$foo: ooh
static::$foo: ooh
$this::$foo: ooh
$this->bar: parent

Class: ChildClass
self::$foo: ooh
static::$foo: ahh
$this::$foo: ahh
$this->bar: child

Setting ParentClass::$foo = 'hmm'

Class: ParentClass
self::$foo: hmm
static::$foo: hmm
$this::$foo: hmm
$this->bar: parent

Class: ChildClass
self::$foo: hmm
static::$foo: ahh
$this::$foo: ahh
$this->bar: child