global定义变量用法

<?php$a = 1;$b = 2;function Sum(){    global $a, $b;    echo $a+$b;}Sum(); //输出3?>

<?php$a = 1;$b = 2;function Sum(){    global $a, $b;    echo $a+$b;}$a = 20;Sum(); //输出22?>

<?php$a = 1;$b = 2;echo '使用global之前'.$a.'</br>';function Sum(){    global $a, $b;    echo $a+$b;}Sum(); //使用global之前1      3?>