Can I access discriminator field from php in doctrine2?

Here is how I'd do.

First, you made an AttributeInterface, to be sure that all future new Attribute types will implement the need method :

interface AttributeInterface
{
    /**
     * Return the attribute type
     */
    public function getType();
}

Then you create the Attribute abstract class implementing the AttributeInterface interface.

Use the constants in the @DiscrimatorMap call for some consistency

/**
 * Attribute
 * ...
 * @DiscriminatorColumn(name="type", type="string")
 * @DiscriminatorMap({Attribute::TYPE_TEXT = "TextAttribute", Attribute::TYPE_BOOLEAN = "BooleanAttribute", Attribute::TYPE_NUMERIC = "NumericAttribute", Attribute::TYPE_DATE = "DateAttribute"})
 */
abstract class Attribute implements AttributeInterface
{
    const TYPE_TEXT    = 'text';
    const TYPE_BOOLEAN = 'boolean';
    const TYPE_NUMERIC = 'numeric';
    const TYPE_DATE    = 'date';
}

Finally, you create all needed classes, extending Attribute class and implementing the getType() method

/**
 * TextAttribute
 *
 * @ORM\Entity
 */
class TextAttribute extends Attribute
{
    public function getType()
    {
        return $this::TYPE_TEXT;
    }
}

/**
 * BooleanAttribute
 *
 * @ORM\Entity
 */
class BooleanAttribute extends Attribute
{
    public function getType()
    {
        return $this::TYPE_BOOLEAN;
    }
}

/**
 * NumericAttribute
 *
 * @ORM\Entity
 */
class  NumericAttribute extends Attribute
{
    public function getType()
    {
        return $this::TYPE_NUMERIC;
    }
}

/**
 * DateAttribute
 *
 * @ORM\Entity
 */
class DateAttribute extends Attribute
{
    public function getType()
    {
        return $this::TYPE_DATE;
    }
}

// And so on...

It's possible either with the EntityManager or using the DocumentManager.

$documentManager->getClassMetadata(get_class($entity))->discriminatorValue;

Extending what beberlei said, you could declare some constants in the Attribute class, and an abstract getType() function. Then, overload it in every derived attribute class.

Something like:

abstract class Attribute {
    const TYPE_BOOL = 0;
    const TYPE_INT  = 1;
    ...
    abstract public function getType();
}

class BooleanAttribute extends Attribute {
    public function getType() {
        return parent::TYPE_BOOL;
    }
}

My approach is to simply access it's value through the meta data doctrine generates

$cmf = $em->getMetadataFactory();
$meta = $cmf->getMetadataFor($class);
$meta->discriminatorValue

will give you the value, so as a method

public static function get_type()
{
    //...get the $em instance 
    $cmf = $em->getMetadataFactory();
    $meta = $cmf->getMetadataFor(__CLASS__);
    return $meta->discriminatorValue;
}

I cache the metadata in a static variable for each class that extends my base entity, there is a lot of other useful information in there as well ...