DTrace Dynamic Tracing
PHP Manual

Using PHP and DTrace

PHP can be configured with DTrace static probes on platforms that support DTrace Dynamic Tracing.

Configuring PHP for DTrace Static Probes

Refer to external platform specific documentation for enabling operating system DTrace support. For example, on Oracle Linux boot a UEK3 kernel and do: 

# modprobe fasttrap
# chmod 666 /dev/dtrace/helper

Instead of using chmod, you could instead use an ACL package rule to limit device access to a specific user.

Build PHP with the --enable-dtrace configuration parameter: 

# ./configure --enable-dtrace ...
# make
# make install

This enables the static probes in core PHP. Any PHP extensions that provide their own probes should be built separately as shared extensions.

DTrace Static Probes in Core PHP

The following static probes are available in PHP
Probe Name Probe Description Probe Arguments
request-startup Fires when a request starts. char *file, char *request_uri, char *request_method
request-shutdown Fires when a request shutdown. char *file, char *request_uri, char *request_method
compile-file-entry Fires when the compilation of a script starts. char *compile_file, char *compile_file_translated
compile-file-return Fires when the compilation of a script finishes. char *compile_file, char *compile_file_translated
execute-entry Fires when an opcode array is to be executed. For example, it fires on function calls, includes, and generator resumes. char *request_file, int lineno
execute-return Fires after execution of an opcode array. char *request_file, int lineno
function-entry Fires when the PHP engine enters a PHP function or method call. char *function_name, char *request_file, int lineno, char *classname, char *scope
function-return Fires when the PHP engine returns from a PHP function or method call. char *function_name, char *request_file, int lineno, char *classname, char *scope
exception-thrown Fires when an exception is thrown. char *classname
exception-caught Fires when an exception is caught. char *classname
error Fires when an error occurs, regardless of the error_reporting level. char *errormsg, char *request_file, int lineno

PHP extensions may also have additional static probes.

Listing DTrace Static Probes in PHP

To list available probes, start a PHP process and then run: 

# dtrace -l

The output will be similar to: 

   ID   PROVIDER            MODULE                          FUNCTION NAME
   [ . . . ]
    4   php15271               php               dtrace_compile_file compile-file-entry
    5   php15271               php               dtrace_compile_file compile-file-return
    6   php15271               php                        zend_error error
    7   php15271               php  ZEND_CATCH_SPEC_CONST_CV_HANDLER exception-caught
    8   php15271               php     zend_throw_exception_internal exception-thrown
    9   php15271               php                 dtrace_execute_ex execute-entry
   10   php15271               php           dtrace_execute_internal execute-entry
   11   php15271               php                 dtrace_execute_ex execute-return
   12   php15271               php           dtrace_execute_internal execute-return
   13   php15271               php                 dtrace_execute_ex function-entry
   14   php15271               php                 dtrace_execute_ex function-return
   15   php15271               php              php_request_shutdown request-shutdown
   16   php15271               php               php_request_startup request-startup

The Provider column values consist of php and the process id of the currently running PHP process.

If the Apache web server is running, the module name might be, for example, libphp5.so, and there would be multiple blocks of listings, one per running Apache process.

The Function column refers to PHP's internal C implementation function names where each provider is located.

If a PHP process is not running, then no PHP probes will be shown.

DTrace with PHP Example

This example shows the basics of the DTrace D scripting language.

Example #1 all_probes.d for tracing all PHP Static Probes with DTrace

#!/usr/sbin/dtrace -Zs

#pragma D option quiet

php*:::compile-file-entry
{
    printf("PHP compile-file-entry\n");
    printf("  compile_file              %s\n", copyinstr(arg0));
    printf("  compile_file_translated   %s\n", copyinstr(arg1));
}

php*:::compile-file-return
{
    printf("PHP compile-file-return\n");
    printf("  compile_file              %s\n", copyinstr(arg0));
    printf("  compile_file_translated   %s\n", copyinstr(arg1));
}

php*:::error
{
    printf("PHP error\n");
    printf("  errormsg                  %s\n", copyinstr(arg0));
    printf("  request_file              %s\n", copyinstr(arg1));
    printf("  lineno                    %d\n", (int)arg2);
}

php*:::exception-caught
{
    printf("PHP exception-caught\n");
    printf("  classname                 %s\n", copyinstr(arg0));
}

php*:::exception-thrown
{
    printf("PHP exception-thrown\n");
    printf("  classname                 %s\n", copyinstr(arg0));
}

php*:::execute-entry
{
    printf("PHP execute-entry\n");
    printf("  request_file              %s\n", copyinstr(arg0));
    printf("  lineno                    %d\n", (int)arg1);
}

php*:::execute-return
{
    printf("PHP execute-return\n");
    printf("  request_file              %s\n", copyinstr(arg0));
    printf("  lineno                    %d\n", (int)arg1);
}

php*:::function-entry
{
    printf("PHP function-entry\n");
    printf("  function_name             %s\n", copyinstr(arg0));
    printf("  request_file              %s\n", copyinstr(arg1));
    printf("  lineno                    %d\n", (int)arg2);
    printf("  classname                 %s\n", copyinstr(arg3));
    printf("  scope                     %s\n", copyinstr(arg4));
}

php*:::function-return
{
    printf("PHP function-return\n");
    printf("  function_name             %s\n", copyinstr(arg0));
    printf("  request_file              %s\n", copyinstr(arg1));
    printf("  lineno                    %d\n", (int)arg2);
    printf("  classname                 %s\n", copyinstr(arg3));
    printf("  scope                     %s\n", copyinstr(arg4));
}

php*:::request-shutdown
{
    printf("PHP request-shutdown\n");
    printf("  file                      %s\n", copyinstr(arg0));
    printf("  request_uri               %s\n", copyinstr(arg1));
    printf("  request_method            %s\n", copyinstr(arg2));
}

php*:::request-startup
{
    printf("PHP request-startup\n");
    printf("  file                      %s\n", copyinstr(arg0));
    printf("  request_uri               %s\n", copyinstr(arg1));
    printf("  request_method            %s\n", copyinstr(arg2));
}

This script uses the -Z option to dtrace, allowing it to be run when there is no PHP process executing. If this option were omitted the script would immediately terminate because it knows none of the probes to be monitored are in existence.

The script traces all core PHP static probe points throughout the duration of a running PHP script. Run the D script: 

# ./all_probes.d

Run a PHP script or application. The monitoring D script will output each probe's arguments as it fires.

When monitoring is complete, the D script can be terminated with a CTRLC.

On multi-CPU machines the probe ordering might not appear to be sequential. This depends on which CPU was processing the probes, and how threads migrate across CPUs. Displaying probe time stamps will help reduce confusion, for example: 

php*:::function-entry
{
      printf("%lld: PHP function-entry ", walltimestamp);
      [ . . .]
}