ActualOutput

[ romain @MacBook : /tmp/test-serializer ] php test.php 
PHP Fatal error:  Uncaught exception 'RuntimeException' with message 'There is neither a public getProbe method, nor a public isProbe method in class Media. Please specify which public method should be used for retrieving the value of the property probe.' in /private/tmp/test-serializer/vendor/jms/serializer/src/JMS/Serializer/Metadata/PropertyMetadata.php:60
Stack trace:
#0 /private/tmp/test-serializer/vendor/jms/serializer/src/JMS/Serializer/Metadata/Driver/AnnotationDriver.php(188): JMS\Serializer\Metadata\PropertyMetadata->setAccessor('public_method', NULL, NULL)
#1 /private/tmp/test-serializer/vendor/jms/metadata/src/Metadata/MetadataFactory.php(77): JMS\Serializer\Metadata\Driver\AnnotationDriver->loadMetadataForClass(Object(ReflectionClass))
#2 /private/tmp/test-serializer/vendor/jms/serializer/src/JMS/Serializer/GraphNavigator.php(177): Metadata\MetadataFactory->getMetadataForClass('Media')
#3 /private/tmp/test-serializer/vendor/jms/serializer/src/JMS/Serializer/Serializer.php(125): JMS\Serializer\GraphNavigator->accep in /private/tmp/test-serializer/vendor/jms/serializer/src/JMS/Serializer/Metadata/PropertyMetadata.php on line 60

Fatal error: Uncaught exception 'RuntimeException' with message 'There is neither a public getProbe method, nor a public isProbe method in class Media. Please specify which public method should be used for retrieving the value of the property probe.' in /private/tmp/test-serializer/vendor/jms/serializer/src/JMS/Serializer/Metadata/PropertyMetadata.php on line 60

RuntimeException: There is neither a public getProbe method, nor a public isProbe method in class Media. Please specify which public method should be used for retrieving the value of the property probe. in /private/tmp/test-serializer/vendor/jms/serializer/src/JMS/Serializer/Metadata/PropertyMetadata.php on line 60

Call Stack:
    0.0005     286512   1. {main}() /private/tmp/test-serializer/test.php:0
    0.0167    2135488   2. JMS\Serializer\Serializer->serialize() /private/tmp/test-serializer/test.php:93
    0.0173    2230496   3. JMS\Serializer\GraphNavigator->accept() /private/tmp/test-serializer/vendor/jms/serializer/src/JMS/Serializer/Serializer.php:125
    0.0176    2255304   4. Metadata\MetadataFactory->getMetadataForClass() /private/tmp/test-serializer/vendor/jms/serializer/src/JMS/Serializer/GraphNavigator.php:177
    0.0177    2258320   5. JMS\Serializer\Metadata\Driver\AnnotationDriver->loadMetadataForClass() /private/tmp/test-serializer/vendor/jms/metadata/src/Metadata/MetadataFactory.php:77
    0.0263    2559824   6. JMS\Serializer\Metadata\PropertyMetadata->setAccessor() /private/tmp/test-serializer/vendor/jms/serializer/src/JMS/Serializer/Metadata/Driver/AnnotationDriver.php:188

ExpectedOutput

[ romain @MacBook : /tmp/test-serializer ] php test.php 
{"path":"photo01.JPG","width":3264,"height":2448}

test.php

<?php

use JMS\Serializer\Annotation\AccessType;
use JMS\Serializer\Annotation\ReadOnly;
use JMS\Serializer\Annotation\ExclusionPolicy;
use JMS\Serializer\Annotation\Exclude;
use JMS\Serializer\Annotation\Expose;
use Doctrine\Common\Annotations\AnnotationRegistry;

include 'vendor/autoload.php';

AnnotationRegistry::registerAutoloadNamespace(
    'JMS\Serializer\Annotation', __DIR__.'/vendor/jms/serializer/src'
);

$serializer = JMS\Serializer\SerializerBuilder::create()
    ->setCacheDir(__DIR__ . '/cache')
    ->setDebug(true)
    ->build();

/**
 * @ExclusionPolicy("all")
 * @AccessType("public_method")
 */
class Media
{
    /**
     * @Exclude
     */
    private $probe;
    /**
     * @Expose
     * @Readonly
     */
    private $path;
    /**
     * @Expose
     * @Readonly
     */
    private $width;
    /**
     * @Expose
     * @Readonly
     */
    private $height;

    public function __construct($probe, $path)
    {
        $this->path = $path;
        $this->probe = $probe;
    }

    public function getPath()
    {
        return $this->path;
    }

    public function getWidth()
    {
        return $this->probe->execute('width', $this->path);
    }

    public function getHeight()
    {
        return $this->probe->execute('height', $this->path);
    }

    public static function create($path)
    {
        return new static(new Probe(), $path);
    }
}

class Probe
{
    public function execute($what, $path)
    {
        $dim = getimagesize($path);

        switch ($what) {
            case 'width':
                return $dim[0];
                break;
            case 'height':
                return $dim[1];
                break;
            default:
                throw new \RuntimeException(sprintf('Unknown %s parameter', $what));
        }
    }
}

print($serializer->serialize(Media::create('photo01.JPG'), 'json')) . "\n";