src/Type/Definition/StringType.php
public function serialize($value)
    {
        if ($value === true) {
            return 'true';
        }
        if ($value === false) {
            return 'false';
        }
        if ($value === null) {
            return 'null';
        }
        if (is_object($value) && method_exists($value, '__toString')) {
            return (string) $value;
        }
        if (! is_scalar($value)) {
            throw new Error('String cannot represent non scalar value: ' . Utils::printSafe($value));
        }

        return $this->coerceString($value);
    }
public function serialize($value)
    {
        if ($value === true) {
            return 'true';
        }
        if ($value === false) {
            return 'false';
        }
        if ($value === null) {
            return 'null';
        }
        if (is_object($value) && method_exists($value, '__toString')) {
            return (string) $value;
        }
        if (! is_scalar($value)) {
            throw new Error('String cannot represent non scalar value: ' . Utils::printSafe($value));
        }

        return $this->coerceString($value);
    }
@param mixed $value

@return mixed|string

@throws Error