ParagonIE_Sodium_Core_Util::substr( string $str, int $start = 0, int $length = null ): string

836    public static function substr($str, $start = 0, $length = null)837    {838        /* Type checks: */839        if (!is_string($str)) {840            throw new TypeError('String expected');841        }842 843        if ($length === 0) {844            return '';845        }846 847        if (self::isMbStringOverride()) {848            if (PHP_VERSION_ID < 50400 && $length === null) {849                $length = self::strlen($str);850            }851            $sub = (string) mb_substr($str, $start, $length, '8bit');852        } elseif ($length === null) {853            $sub = (string) substr($str, $start);854        } else {855            $sub = (string) substr($str, $start, $length);856        }857        if ($sub !== '') {858            return $sub;859        }860        return '';861    }