php 一个无限级数组返回完整的键值

<?php
$array= array('0'=>array('0'=>array('a'=>'我','1'=>'是','b'=>'谁')),'a'=>'wowo','b'=>array('c'=>array('ta','是','谁')));
function print_r_string($arr,$first=true,$tab=0)
{
$output = "";
if ($first) $output .= "<pre><br>\n";
foreach($arr as $key => $val)
{
switch (gettype($val))
{
case "array":
$output .= "[".htmlspecialchars($key)."]";
$tab++;
$output .= print_r_string($val,false,$tab);
break;
case "string":
$output .= "[".htmlspecialchars($key)."]<br>\n";
break;
}
}
if ($first) $output .= "</pre><br>\n";
return $output;
}
echo print_r_string($array);
?>