detect closure in __sleep to prevent their serialization

Question

When I'm trying to serialize an object which has members including closures an exception is thrown. To avoid the serialization of the members including closures I tried the following:

function __sleep(){
    $ref = new ReflectionClass($this);
    $props = $ref->getProperties();

    foreach ($props as $prop){
        $name = $prop->name;
        if (is_callable($this->$name)===false){
            $dream[] = $prop->name;
        }
    }
    return $dream;
}

Unfortunately this does not work. Is there a better way to detect whether a property is a closure or not.

EDIT: I solved my problem by letting the closure know whether to serialize or not

To do this I am wrapping the closure itself. Here's an example:

/**
 * Wrapper-class to prevent closure to be serialized.
 */
class WrappedClosure {

    private $closure = NULL;
    protected $reflection = NULL;

    public function __construct($function){
        if ( ! $function instanceOf Closure)
            throw new InvalidArgumentException();
        $this->closure = $function;
        $this->reflection = new ReflectionFunction($function);
    }

    /**
     * When the instance is invoked, redirect invocation to closure.
     */
    public function __invoke(){
        $args = func_get_args();
        return $this->reflection->invokeArgs($args);
    }

    // do nothing on serialization
    public function __sleep(){}

    // do nothing on serialization
    public function __wakeup(){}
}

// Assigning a wrapped closure to a member
$myObject->memberHoldingAClosure = 
    // Wrapping the closure 
    new WrappedClosure(
        function (){
            echo "I'am the inner closure.";
        }
    )
);

// the serialization doesn't throw an exception anymore
serialize($myObject);

Answer 1

Works fine for me:

class foo {
    protected $param = 'value';
    protected $closure = null;
    public function __construct() {
        $this->closure = function(){
            return 123;
        };
    }
    public function __sleep() {
        $serializable = array();
        foreach ( $this as $paramName => $paramValue ) {
            if ( !is_string($paramValue) && !is_array($paramValue) && is_callable($paramValue) ) {
                continue;
            }
            $serializable[] = $paramName;
        }
        return $serializable;
    }
}
$foo = new foo();
echo serialize($foo);

About checking if value is instance of Closure class (from manual):

Anonymous functions are currently implemented using the Closure class. This is an implementation detail and should not be relied upon.

Therefore I would implement is_closure($value) function as return !is_string($value) && !is_array($value) && is_callable($value) rather than return $value instanceof Closure and hope that some day PHP developers will add native is_closure() function.