foreach问题求助！

把

<?php
$arr=array("one", "two", "three");

foreach ($arr as $a)
{
   echo "Value: " . $a . "<br />";
}
?>


改为

<?php
$arr=array("one", "two", "three");

foreach ($arr)
{
   echo "Value: " . $arr . "<br />";
}
?>
为什么会出错？为什么一定要加上as