gistfile1.php

<?php

class  ParentClass {
	protected $a = 1;
	public function doSomething() {
		echo "I'm the parent";
		$this->a = 2;
	}
}

class childClass {
	public function doSomething() {
		ParentClass::doSomething();
		echo "I'm the child";
		echo $this->a;
	}
}

$c = new childClass();

$c->doSomething();

Surprising output!

Try to remove line 13.

What's so surprising about the result?