php debug print stack

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

rrree

result

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