php 调试打印堆栈

<?php
// filename: /tmp/a.php
function a_test($str)
{
    echo "\nHi: $str";
    var_dump(debug_backtrace());
}
a_test(&#39;friend&#39;);
?>

<?php
// filename: /tmp/b.php
include_once &#39;/tmp/a.php&#39;;
?>

Hi: friend
array(2) {
[0]=>
array(4) {
    ["file"] => string(10) "/tmp/a.php"
    ["line"] => int(10)
    ["function"] => string(6) "a_test"
    ["args"]=>
    array(1) {
      [0] => &string(6) "friend"
    }
}
[1]=>
array(4) {
    ["file"] => string(10) "/tmp/b.php"
    ["line"] => int(2)
    ["args"] =>
    array(1) {
      [0] => string(10) "/tmp/a.php"
    }
    ["function"] => string(12) "include_once"
  }
}