Answers by Sentry

How do I print the call stack in PHP?

Richard C.

The Problem

When an error occurs in PHP and you want to understand the flow of program logic that occurred, it’s useful to be able to print the call stack — all the functions that were called up to this point.

The Solution

Use debug_print_backtrace();.

Below is a simple example PHP script with three functions called in sequence: A, B, and C. Function C prints the call stack:

<?php
A("a");

function A($a) {
    B();
}

function B() {
    C();
}

function C() {
  debug_print_backtrace();
}
?>

Put this script in a file called test.php and run it with php test.php. The output is:

#0 /home/user/test.php(10): C()
#1 /home/user/test.php(5): B()
#2 /home/user/test.php(19): A('a')

This is the fastest way to print the stack trace.

The A('a') indicates the function A received parameters. You probably don’t need to see this, so to make the output neater use: debug_print_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS);.

To print the full details of every call, including parameter values, use:

var_dump(debug_backtrace());

The output will be:

array(3) {
  [0]=>
  array(4) {
    ["file"]=>
    string(35) "/home/user/test.php"
    ["line"]=>
    int(9)
    ["function"]=>
    string(1) "C"
    ["args"]=>
    array(0) {
    }
  }
  [1]=>
  array(4) {
    ["file"]=>
    string(35) "/home/user/test.php"
    ["line"]=>
    int(5)
    ["function"]=>
    string(1) "B"
    ["args"]=>
    array(0) {
    }
  }
  [2]=>
  array(4) {
    ["file"]=>
    string(35) "/home/user/test.php"
    ["line"]=>
    int(2)
    ["function"]=>
    string(1) "A"
    ["args"]=>
    array(1) {
      [0]=>
      string(1) "a"
    }
  }
}

To ignore function arguments, you can use var_dump(debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS));.

Use An Exception Trace To Reduce Memory Use

Some users on StackOverflow and the PHP manual website say that backtrace can cause memory problems with very large applications. If you encounter this, you could instead try printing the call stack with:

$e = new Exception();
var_dump($e->getTraceAsString());

Further Reading

Loved by over 4 million developers and more than 90,000 organizations worldwide, Sentry provides code-level observability to many of the world’s best-known companies like Disney, Peloton, Cloudflare, Eventbrite, Slack, Supercell, and Rockstar Games. Each month we process billions of exceptions from the most popular products on the internet.

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