PHP 5.5.6(在方法之间交换数据）从getValue方法调用私有方法和静态变量，只有在设置新值后才能工作 - PHP 5.5.6 (exchangeing data between methods) calling private method and static variable from getValue method, works only after setting new value

PHP 5.5.6 (exchangeing data between methods) calling private method and static variable from getValue method, works only after setting new value

请有人向我解释一下这个错误的原因是什么？。。在为$variable 分配新值后，为什么它工作良好

<?php
 class MyClass {
   var $variable = 'first_variable';
   static $qt = 1;
   public function getValue() {
     return $this->variable . ' - calling getValue: ' . MyClass::increment();
   }
   public function setValue($pass) {
     return $this->variable = $pass;
   }
   private function increment() {
     return $this->qt++;
   }
 }
 $ob = new MyClass();
 echo $ob->getValue();
 $ob->setValue('changed_variable');
 echo '<br>' . $ob->getValue();
 echo '<br>' . $ob->getValue();
 echo '<br>' . $ob->getValue();

只对第一个值给出错误，然后运行良好，

错误：注意：第18行C:''examplep''htdocs''somename''1.php中未定义的属性：MyClass:：$qt

first_variable - calling getValue:
changed_variable - calling getValue: 1
changed_variable - calling getValue: 2
changed_variable - calling getValue: 3

在这种情况下，问题是我试图将静态属性作为非静态属性访问。分辨率是

private function increment()
 { 
return MyClass::qt++; 
}