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Advanced Usage of the VarDumper Component

Advanced Usage of the VarDumper Component

The dump() function is just a thin wrapper and a more convenient way to call VarDumper::dump(). You can change the behavior of this function by calling VarDumper::setHandler($callable). Calls to dump() will then be forwarded to $callable.

By adding a handler, you can customize the Cloners, Dumpers and Casters as explained below. A simple implementation of a handler function might look like this:

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use Symfony\Component\VarDumper\VarDumper;
use Symfony\Component\VarDumper\Cloner\VarCloner;
use Symfony\Component\VarDumper\Dumper\CliDumper;
use Symfony\Component\VarDumper\Dumper\HtmlDumper;

VarDumper::setHandler(function ($var) {
    $cloner = new VarCloner();
    $dumper = 'cli' === PHP_SAPI ? new CliDumper() : new HtmlDumper();

    $dumper->dump($cloner->cloneVar($var));
});

Cloners

A cloner is used to create an intermediate representation of any PHP variable. Its output is a Data object that wraps this representation.

You can create a Data object this way:

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use Symfony\Component\VarDumper\Cloner\VarCloner;

$cloner = new VarCloner();
$data = $cloner->cloneVar($myVar);
// this is commonly then passed to the dumper
// see the example at the top of this page
// $dumper->dump($data);

A cloner also applies limits when creating the representation, so that the corresponding Data object could represent only a subset of the cloned variable. Before calling cloneVar(), you can configure these limits:

  • setMaxItems() configures the maximum number of items that will be cloned past the first nesting level. Items are counted using a breadth-first algorithm so that lower level items have higher priority than deeply nested items;
  • setMaxString() configures the maximum number of characters that will be cloned before cutting overlong strings;
  • in both cases, specifying -1 removes any limit.

Before dumping it, you can further limit the resulting Data object by calling its getLimitedClone() method:

  • the first $maxDepth argument allows limiting dumps in the depth dimension,
  • the second $maxItemsPerDepth limits the number of items per depth level,
  • and the last $useRefHandles defaults to true, but allows removing internal objects' handles for sparser output,
  • but unlike the previous limits on cloners that remove data on purpose, these can be changed back and forth before dumping since they do not affect the intermediate representation internally.

Note

When no limit is applied, a Data object is as accurate as the native serialize function, and thus could be for purposes beyond dumping for debugging.

Dumpers

A dumper is responsible for outputting a string representation of a PHP variable, using a Data object as input. The destination and the formatting of this output vary with dumpers.

This component comes with an HtmlDumper for HTML output and a CliDumper for optionally colored command line output.

For example, if you want to dump some $variable, just do:

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use Symfony\Component\VarDumper\Cloner\VarCloner;
use Symfony\Component\VarDumper\Dumper\CliDumper;

$cloner = new VarCloner();
$dumper = new CliDumper();

$dumper->dump($cloner->cloneVar($variable));

By using the first argument of the constructor, you can select the output stream where the dump will be written. By default, the CliDumper writes on php://stdout and the HtmlDumper on php://output. But any PHP stream (resource or URL) is acceptable.

Instead of a stream destination, you can also pass it a callable that will be called repeatedly for each line generated by a dumper. This callable can be configured using the first argument of a dumper's constructor, but also using the setOutput() method or the second argument of the dump() method.

For example, to get a dump as a string in a variable, you can do:

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use Symfony\Component\VarDumper\Cloner\VarCloner;
use Symfony\Component\VarDumper\Dumper\CliDumper;

$cloner = new VarCloner();
$dumper = new CliDumper();
$output = '';

$dumper->dump(
    $cloner->cloneVar($variable),
    function ($line, $depth) use (&$output) {
        // A negative depth means "end of dump"
        if ($depth >= 0) {
            // Adds a two spaces indentation to the line
            $output .= str_repeat('  ', $depth).$line."\n";
        }
    }
);

// $output is now populated with the dump representation of $variable

Another option for doing the same could be:

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use Symfony\Component\VarDumper\Cloner\VarCloner;
use Symfony\Component\VarDumper\Dumper\CliDumper;

cloner = new VarCloner();
$dumper = new CliDumper();
$output = fopen('php://memory', 'r+b');

$dumper->dump($cloner->cloneVar($variable), $output);
$output = stream_get_contents($output, -1, 0);

// $output is now populated with the dump representation of $variable

Dumpers implement the DataDumperInterface interface that specifies the dump(Data $data) method. They also typically implement the DumperInterface that frees them from re-implementing the logic required to walk through a Data object's internal structure.

Casters

Objects and resources nested in a PHP variable are "cast" to arrays in the intermediate Data representation. You can tweak the array representation for each object/resource by hooking a Caster into this process. The component already includes many casters for base PHP classes and other common classes.

If you want to build your own Caster, you can register one before cloning a PHP variable. Casters are registered using either a Cloner's constructor or its addCasters() method:

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use Symfony\Component\VarDumper\Cloner\VarCloner;

$myCasters = array(...);
$cloner = new VarCloner($myCasters);

// or

$cloner->addCasters($myCasters);

The provided $myCasters argument is an array that maps a class, an interface or a resource type to a callable:

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$myCasters = array(
    'FooClass' => $myFooClassCallableCaster,
    ':bar resource' => $myBarResourceCallableCaster,
);

As you can notice, resource types are prefixed by a : to prevent colliding with a class name.

Because an object has one main class and potentially many parent classes or interfaces, many casters can be applied to one object. In this case, casters are called one after the other, starting from casters bound to the interfaces, the parents classes and then the main class. Several casters can also be registered for the same resource type/class/interface. They are called in registration order.

Casters are responsible for returning the properties of the object or resource being cloned in an array. They are callables that accept four arguments:

  • the object or resource being casted,
  • an array modelled for objects after PHP's native (array) cast operator,
  • a Stub object representing the main properties of the object (class, type, etc.),
  • true/false when the caster is called nested in a structure or not.

Here is a simple caster not doing anything:

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function myCaster($object, $array, $stub, $isNested)
{
    // ... populate/alter $array to your needs

    return $array;
}

For objects, the $array parameter comes pre-populated using PHP's native (array) casting operator or with the return value of $object->__debugInfo() if the magic method exists. Then, the return value of one Caster is given as the array argument to the next Caster in the chain.

When casting with the (array) operator, PHP prefixes protected properties with a \0*\0 and private ones with the class owning the property. For example, \0Foobar\0 will be the prefix for all private properties of objects of type Foobar. Casters follow this convention and add two more prefixes: \0~\0 is used for virtual properties and \0+\0 for dynamic ones (runtime added properties not in the class declaration).

Note

Although you can, it is advised to not alter the state of an object while casting it in a Caster.

Tip

Before writing your own casters, you should check the existing ones.

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.