PHP RFC: debug_backtrace_depth(int $limit=0): int


Inspecting the current stack trace depth is occasionally useful for manually debugging, checking for potential infinite recursion, or for checking if code would hit stack frame limits in extensions such as Xdebug (xdebug.max_nesting_limit). It is currently possible to compute the depth through count(debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, $limit=0)), but this is verbose and inefficient compared to returning the depth directly.


Add a new function debug_backtrace_depth(int $limit=0): int that will compute the depth directly rather than build an array with the frame data.

This can be polyfilled with min($limit, count(debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS))). or less accurately as count(debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, $limit))

  • For debug_backtrace_depth, the limit is on the returned depth. If that depth is exceeded, $limit is returned instead)
  • For debug_backtrace, the preexisting behavior is that the $limit is on the number of frames scanned, not the size of the returned array, and this may differ. Changing this to be a limit on the returned depth may be an improvement, but is out of scope for this RFC.

When $limit <= 0, there is no limit.

Internally, PHP's stack frames are represented like a singly linked list (i.e. to count them, you need to iterate through the frames), and this RFC does not change that representation. The amount of time needed to compute the depth is proportional to the depth, which is why this RFC supports the optional $limit. Counting the frames is still much faster than constructing an array with the file, line, class, etc. with debug_backtrace().

Example use cases

While it is possible to do these things already with count(debug_backtrace(...)), having a dedicated function to do this makes it more concise, efficient, and readable.

Tracking down potentially infinite recursion

function fibonacci(int $n): int|float {
    if (debug_backtrace_depth(1000) >= 1000) {  // temporary debugging code
        throw new RuntimeException("Possibly infinite recursion at n=$n - was a base case forgotten?");
    if ($n === 1) { return 1; }
    return fibonacci($n - 2) + fibonacci($n - 1);
echo fibonacci(10);
Warning: Uncaught RuntimeException: Possibly infinite recursion at n=-1988 - was a base case forgotten? in php shell code:3                                          
Stack trace:
#0 php shell code(7): fibonacci(-1988)
#1 php shell code(7): fibonacci(-1986)

Checking for compatibility with extensions with stack trace limits

For example, in Xdebug https://xdebug.org/docs/all_settings#max_nesting_level has a default value of 256 - if you are developing an application or library which cannot override max_nesting_level you may want to ensure your library does not use deep recursion.

// e.g. a logging function
function my_common_function() {
    if (is_running_as_unit_test() && debug_backtrace_depth(250) >= 250) {
        throw new TestFrameworkError("Refactor this code, xdebug.max_nesting_level may be reached in other environments");
    // body of my_common_function()


This makes it more convenient to efficiently visualize how deep recursion is when adding temporary debugging statements, or where recursive calls are made relative to other calls.

function fibonacci_debug(int $n) {
    if ($n <= 1) {
        $result = 1;
    } else {
        $result = fibonacci_debug($n - 2) + fibonacci_debug($n - 1);
    printf("%sfib(%d) = %d\n", str_repeat('|', debug_backtrace_depth() - 1), $n, $result);
    return $result;
||fib(0) = 1
||fib(1) = 1
|fib(2) = 2
||fib(1) = 1
|||fib(0) = 1
|||fib(1) = 1
||fib(2) = 2
|fib(3) = 3
fib(4) = 5

RFC Impact


None, other than creating the new global function debug_backtrace_depth

Open Issues

Make sure there are no open issues when the vote starts!

Unaffected PHP Functionality

debug_backtrace and debug_print_backtrace are unchanged.

Future Scope

The behavior of debug_backtrace and debug_print_backtrace could be changed to make $limit be consistent with the documentation in a separate RFC. https://www.php.net/manual/en/function.debug-backtrace.php#refsect1-function.debug-backtrace-parameters.

Proposed PHP Version



Limited to a specific use case


There hasn't been much discussion on this one, probably because the functionality and use cases are so specific. I'm not really convinced by your examples.

For the purpose of printing debug logs, using an engine-provided depth is non-ideal, because any intermediate helper functions will count towards the depth. Tracking your own depth will result in a more meaningful/predictable output.

For tracking down infinite recursion, isn't this what the xdebug recursion limit is for? You'll directly get your recursive stack trace, which should make it obvious where the infinite recursion occurs. Using debug_backtrace_depth() for this purpose means that you actually already need to know where you are infinitely recursing in order to place the guard.

If you want to find places where your code would hit the xdebug recursion limit, then why not run it under xdebug and find out, rather than trying to simulate the same behavior manually? That again requires that you actually place relevant guards in the first place, which makes this another chicken and egg problem.

Proposed Voting Choices

Yes/No, requiring a 2/3 majority

rfc/debug_backtrace_depth.txt · Last modified: 2021/06/02 02:05 by tandre