PHP Decimal class

<?php

class DecimalException extends \Exception {}

// NOTE: math operators (neg, mul, div, sub, add) creates new object for result
class Decimal implements \JsonSerializable
{
    const COMPARE_EQUAL   = 0;
    const COMPARE_GREATER = 0.1;
    const COMPARE_LESS    = -0.1;

    private static $zero;

    /**
     * @var resource GMP Integer
     */
    private $value;

    /**
     * @param int|string $value Value to be converted with GMP.
     * @param int        $radix The radix
     *
     * @throws DecimalException If value could not benn converted to GMP resource
     */
    public function __construct($value, $radix = 10)
    {
        $this->value = @gmp_init($value, $radix);

        if (false === $this->value) {
            throw new DecimalException("Wrong value format or type", $value);
        }

    }

    public function isLessThan(Decimal $value)
    {
        return (gmp_cmp($this->value, $value->value) < 0);
    }

    public function isEqualTo(Decimal $value)
    {
        return !gmp_cmp($this->value, $value->value); // == 0
    }

    public function isGreaterThan(Decimal $value)
    {
        return (gmp_cmp($this->value, $value->value) > 0);
    }

    public function isZero()
    {
        return !gmp_intval($this->value); // gmp_intval should be faster than comparison
    }

    public function neg()
    {
        $_ret = new Decimal(0);

        $_ret->value = gmp_neg($this->value);

        return $_ret;
    }

    public function abs()
    {
        $_ret = new Decimal(0);

        $_ret->value = gmp_abs($this->value);

        return $_ret;
    }

    public function add(Decimal $value)
    {
        $_ret = new Decimal(0);

        $_ret->value = gmp_add($this->value, $value->value);

        return $_ret;
    }

    public function sub(Decimal $value)
    {
        $_ret = new Decimal(0);

        $_ret->value = gmp_sub($this->value, $value->value);

        return $_ret;
    }


    public function mul(Decimal $value)
    {
        $_ret = new Decimal(0);

        $_ret->value = gmp_mul($this->value, $value->value);

        return $_ret;
    }

    public function div(Decimal $value, $round = GMP_ROUND_ZERO)
    {
        $_ret = new Decimal(0);

        $_ret->value = gmp_div_q($this->value, $value->value, $round);

        return $_ret;
    }

//    public function div_r(Decimal $value)
//    {
//        $_ret = new Decimal(0);
//
//        $_ret->value = gmp_div_r($this->value, $value->value);
//
//        return $_ret;
//    }

    public function __toString()
    {
        return gmp_strval($this->value);
    }

    public static function zero()
    {
        if (!self::$zero) {
            self::$zero = new Decimal(0);
        }

        return self::$zero;
    }

    public function jsonSerialize()
    {
        return (string)$this;
    }
}