php函数变量引用

<?php    $ceshi=1;    class ceshi1 {        public function ceshi1 (){            global $ceshi;            echo $ceshi;        }    }    class ceshi2{        private $ceshi2;        public function __construct(){            $this->ceshi2=2;        }        public function ceshi2(){            echo $this->ceshi2;        }    }    $ceshi1=new ceshi1;    $ceshi1->ceshi1();    echo '</br>';    $ceshi2=new ceshi2;    $ceshi2->ceshi2();?>