What is PHP doing?

Sometimes when you have a long running PHP script, you might wonder what the hell it is doing at the moment. There are a few tools that can help you to find out, without having to stop the script. Some of these work only on Linux.


The first tool that you can use is strace. strace is a tool that traces system calls. System calls are made by PHP for reading/writing from/to the network and/or files; this also includes reading and writing to databases over the network or Unix domain sockets. strace will also show other system calls such as time. You can use strace by running something like:

strace -p <processid>

The processid you can figure out by running ps aux | grep php and then guessing the correct one from the list - probably the one that uses 100% CPU time. It is also possible to run strace directly with your program's arguments:

strace php yourscript.php

You can even restrict the output to only show certain system calls. With the following command it will only show the open and close system calls:

strace -e open,close php yourscript.php

The output of that last call, with the yourscript.php file not existing, has the following bits in it:

open("/lib/x86_64-linux-gnu/libnss_files.so.2", O_RDONLY) = 3
close(3)                                = 0
open("/etc/services", O_RDONLY|O_CLOEXEC) = 3
close(3)                                = 0
open("/sys/devices/system/cpu", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 3
close(3)                                = 0
open("yourscript.php", O_RDONLY)        = -1 ENOENT (No such file or directory)
Could not open input file: yourscript.php

It is also very useful to find out which php.ini files PHP is trying to load:

strace -e open php yourscript.php 2>&1 | grep php.ini

Grepping needs to be done for the stderr file descriptor, hence the slightly cryptic 2>&1 | grep php.ini.

In my case, running this shows:

derick@whisky:/tmp$ strace -e open php yourscript.php 2>&1 | grep php.ini
open("/usr/local/php/5.3dev/bin/php.ini", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/local/php/5.3dev/lib/php.ini", O_RDONLY) = 3


The ltrace tool can be run and used in the same way as strace, and will tell you which intra-library functions are being called. It produces a lot more information than strace but it should give a good hint of all the C-library functions that are being called. Not as useful as strace though.


If neither strace or ltrace produce any output, it's likely that your script or extension is doing lots of computing. To then figure out in which PHP function your script currently is, you can use the gdb tool.

gdb—the GNU debugger—is a general purpose debugger for C/C++ applications and provides the same functions as Xdebug provides for PHP script debugging. Again, you can either run gdb with new arguments, and then type on the (gdb) prompt run:

gdb --args php yourscript.php
(gdb) run

Or with an already running process and then running cont on the prompt:

gdb -p <processid>
(gdb) cont

Once the script gets to a state in which you are interested to see what it is doing, you can press Ctrl-C. At that moment you come back at the (gdb) prompt where you can run bt to get a list of all the C-level functions that PHP has been calling. For example:

Program received signal SIGINT, Interrupt.
xdebug_execute (op_array=0x1f03b00)
        at /home/derick/dev/php/xdebug/xdebug.c:1409
1409                    zval_ptr_dtor(EG(return_value_ptr_ptr));
(gdb) bt
#0  xdebug_execute (op_array=0x1f03b00)
        at /home/derick/dev/php/xdebug/xdebug.c:1409
#1  0x000000000099d9a2 in zend_do_fcall_common_helper_SPEC (
        at /home/derick/dev/php/php-src-git/branches/PHP_5_3/Zend/zend_vm_execute.h:344
#2  0x00000000009a1cb7 in ZEND_DO_FCALL_SPEC_CONST_HANDLER (
        at /home/derick/dev/php/php-src-git/branches/PHP_5_3/Zend/zend_vm_execute.h:1640
#3  0x000000000099cce7 in execute (op_array=0x1ee2d50)
        at /home/derick/dev/php/php-src-git/branches/PHP_5_3/Zend/zend_vm_execute.h:107
#4  0x00007f3de53debfe in xdebug_execute (op_array=0x1ee2d50)
        at /home/derick/dev/php/xdebug/xdebug.c:1391
#5  0x00000000009696a8 in zend_execute_scripts (type=8, retval=0x0,
        at /home/derick/dev/php/php-src-git/branches/PHP_5_3/Zend/zend.c:1236
#6  0x00000000008f296d in php_execute_script (primary_file=0x7fff9ad6d8b0)
        at /home/derick/dev/php/php-src-git/branches/PHP_5_3/main/main.c:2308
#7  0x0000000000a4b8b3 in main (argc=2, argv=0x7fff9ad6db48)
        at /home/derick/dev/php/php-src-git/branches/PHP_5_3/sapi/cli/php_cli.c:1189

In order to see in which PHP function you are, bt is not very useful. For every internal function or method, you will see a function starting with zif_ or zim_, but user defined functions will only have an execute function listed. You can however find out which user defined functions are being run as well, by means of the .gdbinit script.

This script is part of the PHP source distribution and lives in the top level directory of the source tree. Because I always want this file to load, I have copied this file to my home directory to live as /home/derick/.gdbinit. gdb will always load this file when it starts, but if you only want to do this for specific cases, you can do that by running on the (gdb) prompt:

(gdb) source /home/derick/dev/php/PHP_5_3/.gdbinit

In either case, with the script loaded you can then run on the (gdb) prompt:

(gdb) zbacktrace

With this simple script:

function addOne( &$i )

$i = 0;
while (true) {
        addOne( $i );

This command should (almost) always show:

(gdb) zbacktrace
[0xec906090] addOne() /tmp/yourscript.php:9

Hope this helps!


This article has a short URL available: https://drck.me/phpwhat-9h9


The .gdbinit trick was something I didn't know before. Very, very helpful.

One thing I can never manage is getting reproducible cores with Apache - thus, many segfaults, although reproducible, can hardly be investigated (because starting up Apache in single thread mode on a live website is not really the way to go).

Do you know a reliable way of getting Apache2 to throw core?

Oh, ltrace - hadn't heard of that before. I've used strace a number of times in the past.


About the php.ini thing with strace, what's the difference with just running something like php -i | grep -E '\.ini' ?

@Geoffrey: It sometimes lies.

I think "Some of these work only on Linux" might be an understatement, as DBG seems to be the only application that has a Windows port (WinDBG).

Personally though, I've found some basic value in both ProcessExplorer and ProcessMonitor from Sysinternals.

great post, keep these post (on debugging php) coming!

Hehe, very nice post and good ideas :)

I have used strace before but never played with gdb, seems like a nice way to do it with .gdbinit.

Thanks a lot


if you use .gdbinit + zbacktrace from php,make sure to have the debugsymbols for your php version installed. Probably php?-cli-dbgsym, otherwise you get

(gdb) zbacktrace Attempt to extract a component of a value that is not a structure.

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