Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - php-amqplib/php-amqplib v2.6.3 requires ext-bcmath * -> the requested PHP extension bcmath is missing from your system.
    - php-amqplib/php-amqplib v2.6.2 requires ext-bcmath * -> the requested PHP extension bcmath is missing from your system.
    - php-amqplib/php-amqplib v2.6.1 requires ext-bcmath * -> the requested PHP extension bcmath is missing from your system.
    - php-amqplib/php-amqplib v2.6.0 requires ext-bcmath * -> the requested PHP extension bcmath is missing from your system.
    - Installation request for php-amqplib/php-amqplib ^2.6 -> satisfiable by php-amqplib/php-amqplib[v2.6.0, v2.6.1, v2.6.2, v2.6.3].

  To enable extensions, verify that they are enabled in your .ini files:
    - /etc/php/7.0/cli/php.ini
    - /etc/php/7.0/cli/conf.d/10-mysqlnd.ini
    - /etc/php/7.0/cli/conf.d/10-opcache.ini
    - /etc/php/7.0/cli/conf.d/10-pdo.ini
    - /etc/php/7.0/cli/conf.d/20-calendar.ini
    - /etc/php/7.0/cli/conf.d/20-ctype.ini
    - /etc/php/7.0/cli/conf.d/20-exif.ini
    - /etc/php/7.0/cli/conf.d/20-fileinfo.ini
    - /etc/php/7.0/cli/conf.d/20-ftp.ini
    - /etc/php/7.0/cli/conf.d/20-gettext.ini
    - /etc/php/7.0/cli/conf.d/20-iconv.ini
    - /etc/php/7.0/cli/conf.d/20-json.ini
    - /etc/php/7.0/cli/conf.d/20-mysqli.ini
    - /etc/php/7.0/cli/conf.d/20-pdo_mysql.ini
    - /etc/php/7.0/cli/conf.d/20-phar.ini
    - /etc/php/7.0/cli/conf.d/20-posix.ini
    - /etc/php/7.0/cli/conf.d/20-readline.ini
    - /etc/php/7.0/cli/conf.d/20-shmop.ini
    - /etc/php/7.0/cli/conf.d/20-sockets.ini
    - /etc/php/7.0/cli/conf.d/20-sysvmsg.ini
    - /etc/php/7.0/cli/conf.d/20-sysvsem.ini
    - /etc/php/7.0/cli/conf.d/20-sysvshm.ini
    - /etc/php/7.0/cli/conf.d/20-tokenizer.ini
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

Installation failed, deleting ./composer.json. 

You can install it depending on what OS you are using:

Ubuntu

sudo apt install php7.0-bcmath

CentOS

yum install bcmath
>>>> yum install php-bcmath