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Variable tracing with Xdebug

Some time ago Matthew mentioned on IRC that he'd like to see variable modifications in Xdebug's function traces . After I had a quick look at the feasibility of this feature I spend some time on implementing it for Xdebug's HEAD branch that is going to become Xdebug 2.1.

Variable modification tracing can be enabled by setting the php.ini xdebug.collect_assignments setting to 1. Of course this can also be done in either .htaccess or by using ini_set(). This setting requires general execution tracing to be enabled as well and it's only available for human readable trace files (the default format).

If variable modification tracing is turned on, all variable assignments, as well as the update-in-place operators such as += and *=, are being logged to the trace file. Take for example the following script:

1  <?php
2  function multiply( $n, $multiplier )
3  {
4      $val = $n;
5      $val *= $multiplier;
6      return $val;
7  }
9  $a = 3;
10 $a = multiply( $a, 14 );
11 ?>

There are multiple statements that modify variables (on lines 4, 5, 9 and 10). Those statements generate lines in the execution trace file. Combined with turning on xdebug.collect_params and xdebug.collect_return the trace file then turns out to be:

TRACE START [2009-03-20 19:20:38]
    0.0003     119632   -> {main}() /tmp/example1.php:0
                         => $a = 3 /tmp/example1.php:9
    0.0004     120104     -> multiply(3, 14) /tmp/example1.php:10
                           => $val = 3 /tmp/example1.php:4
                           => $val *= 14 /tmp/example1.php:5
                           >=> 42
                         => $a = 42 /tmp/example1.php:10
                         >=> 1
    0.0007      56360
TRACE END   [2009-03-20 19:20:38]

Of course, this feature also works for updating object's properties and array's elements as can be seen with the following script:

1  <?php
2  class Number
3  {
4      private $n;
6      public function __construct( $n )
7      {
8          $this->n = $n;
9      }
11     public function multiply( $multiplier )
12     {
13         $this->n *= $multiplier;
14     }
15 }
16 $a = new Number( 3 );
17 $a->multiply( 14 );
19 $array['3x14'] = $a;
20 ?>

And this creates the following trace file (without the time and memory columns):

-> {main}() /tmp/example2.php:0
  -> Number->__construct(3) /tmp/example2.php:16
   => $this->n = 3 /tmp/example2.php:8
   >=> NULL
 => $a = class Number { private $n = 3 } /tmp/example2.php:16
  -> Number->multiply(14) /tmp/example2.php:17
   => $this->n *= 14 /tmp/example2.php:13
   >=> NULL
 => $array->3x14 = class Number { private $n = 42 } /tmp/example2.php:19
 >=> 1

At the moment, array keys and object properties both use the -> syntax in the trace files, but this might change in the future. There are also still issues with syntax like $this->foo['key'].


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Very nice featured indeed!

Does this mean that xdebug will soon display variable values inside method/function calls in PHP 5.3? (/me waits with bated breath...)


Very nice addition. Thanks for your work.

nice feature! but is there a mechanism for filtering , so we can trace some variables instead of all vars?

Is there an eta on 2.1 release? I'd LOVE this feature.

@Heu: No, because that'd be really slow.

@Edward: No, no ETA yet. But definitely after PHP 5.3 is released.

Will this enable IDE authors to support setting "watches" on variables? The idea being that I have some variable in some object and I want to set a trigger to when it gets modified, so that I can look from where the change was originally triggered and continue debugging there there.

@Lukas: No, this doesn't directly allow for watches directly but it is definitely something that could be easily added on top of this. Feel free to file this as a feature request at


Hi Derick,

It looks like the xdebug only output the assignment = but not =& . Can you confirm it?

Many thanks, Anh

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