Print PHP running call stack from outside the PHP

Question

Is there any runtime stack trace feature in PHP, by a given PID of it? (For whom also write Java, I mean jstack.)

I got few PHP background process that they are freeze once a while on some unknown lines. I can simply kill them all and restart but that don't prevent it happen again.

Is there a API able to spy the stack and tell? like the jstack utility provided from JDK?

Answer 1

You have a few options in terms of debugging unknown errors.

• The first method is easy, but requires a PHP extension to be installed.
• The second method offers a more difficult hands-on approach, but is definitely worth it if you're into the low-level inner-workings of the PHP interpreter. Also, this will require linux and PHP to be configured with --enable-debug.
• The third and, in my personal opinion the easiest, method doesn't require the use of any external programs or added PHP extensions.

---

1. Xdebug

   • This is a PHP extension you can install and have available for any site/page.
   • A quick-list of features from the website:
     
     • Automatic stack trace upon error
     • Function call logging
     • Display features such as enhanced var_dump() output and code coverage information.
   • If you see yourself needing to debug your scripts a lot, this one might be the best option. The prettier-display of errors is also a good thumbs-up in my book.
     
     

2. Use gdb to run the file that crashes and analyze the backtrace.

   • This is an intermediate-to-advanced approach that requires PHP to be configured with --enable-debug, a linux machine running Apache, and a strong desire/ability to understand the way software works on a lower-level.
   • Run gdb with Apache:
     gdb /usr/lib/httpd
   • Then, you have two options.:
     • Run apache as a server and load the file through a browser, as normal:
       (gdb) run -X
       ... now, in your browser - access the page that crashes as normal and switch back to gdb: (gdb) backtrace
       ... the full backtrace will be printed
     • Or, use gdb to run the script itself:
       (gdb) run /path/to/the/script.php
(gdb) backtrace
       ... the full backtrace will be printed
   • For more gdb info, check out the quick-reference guide.
     
     

3. Create a custom error handler that prints the stack trace when an error is thrown.

   • To be sure to catch all errors, you can catch Errors, Exceptions, and PHP's Shutdown events.
   • To debug a single page, it's as simple as creating a function to handle the error and then pasting that function into the top of the page you're having issues on. One step further, I have an "error handling" class that's in a separate file and just include it whenever needed (simplified-version below). You can tweak each method to suit your display needs, or even handle errors on a per-error-number basis!
   • Then, to use this, just add require('ErrorHandler.php'); to the top of your page and it should auto-register itself to handle any errors. Be sure to update the include-path to point to the actual file, of course.

ErrorHandler.php:

<?php
class ErrorHandler {
    public static function captureError($err_no, $message, $file, $line) {
        echo '<strong>Error (#' . $err_no . '):</strong> ' . $message . ' in ' . $file . ' on line #' . $line . '<br />';
        debug_print_backtrace();
    }

    public static function captureException($exception) {
        echo '<pre>' . print_r($exception, true) . '</pre>';
    }

    public static function captureShutdown() {
        if (($error = error_get_last()) !== null) {
            debug_print_backtrace();
        }
    }
}
set_error_handler(array('ErrorHandler', 'captureError'));
set_exception_handler(array('ErrorHandler', 'captureException'));
register_shutdown_function(array('ErrorHandler', 'captureShutdown'));
?>