通过explode函数分割字符串成数组，怎么让索引从1开始

<?php   
$str = "1,2,3,4,5,6";
$a = explode(',',$str);

$b = Array();
foreach($a as $key=>$val){
    $b[$key+1] = $val;
}
print_r($a);
print_r($b);
?>