听起来你的包含顺序可能有点不对劲.

更新：

如果我理解正确你有这样的事情：

header.PHP文件：

<?PHP
echo "In header.PHP\n";

require_once("navigation.PHP");
require_once("backnext.PHP");

echo "Also seen in header.PHP:\n";
echo "prev=$prev\n";
echo "next=$next\n";
?>

navigation.PHP：

<?PHP
echo "In navigation.PHP\n";
$prev = "Hello World#1";
$next = "Hello World#2";
echo "Exiting navigation.PHP\n";
?>

backnext.PHP：

<?PHP
echo "In backnext.PHP\n";
echo "prev=$prev\n";
echo "next=$next\n";
echo "Exiting backnext.PHP\n";
?>

如果我运行这个我得到：

In header.PHP
In navigation.PHP
Exiting navigation.PHP
In backnext.PHP
prev=Hello World#1
next=Hello World#2
Exiting backnext.PHP
Also seen in header.ph
prev=Hello World#1
next=Hello World#2

backnext.PHP能够看到$prev和$next.