PHP 全局变量在用户自定义函数内部不足见

<?php
$val1 = 1;
$val2 = 2;
function test()
{
   echo "start\n";
   echo "val1: ".$val1."\n";                                // 不可见
      echo "val2: ".$val2."\n";                             // 不可见

   global $val2;                                    //global声明了$val2， 使其可见
   echo "声明：global val2 以后\n"; 
      echo "val2: ".$val2."\n";                                //$val2 可见了
   $val2 = 100; 
   echo "end\n";
   $val1 = 100;                                                    //只是局部变量
}
test();
echo "调用了test()以后:\nval2 = ".$val2."\n";      //$val2被test()函数改变了
echo "val1 = ".$val1."\n";                                    //没有被test()改变
?>