Composer not installing dependencies into PHP project

Question

I'm trying to add the Paypal PHP SDK to my project:

composer require "paypal/rest-api-sdk-php:*"

but I keep getting the following error, and composer doesn't add \\vendor into my project.

user@user:/opt/lampp/htdocs/projects/paypal/ipn$ composer require "paypal/rest-api-sdk-php:*"
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - The requested PHP extension ext-curl * is missing from your system. Install or enable PHP's curl extension.
  Problem 2
    - phpunit/phpunit 3.7.9 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.8 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.7 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.6 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.5 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.4 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.38 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.37 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.36 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.35 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.34 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.33 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.32 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.31 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.30 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.3 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.29 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.28 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.27 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.26 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.25 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.24 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.23 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.22 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.21 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.20 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.2 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.19 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.18 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.17 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.16 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.15 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.14 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.13 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.12 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.11 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.10 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - Installation request for phpunit/phpunit 3.7.* -> satisfiable by phpunit/phpunit[3.7.0, 3.7.1, 3.7.10, 3.7.11, 3.7.12, 3.7.13, 3.7.14, 3.7.15, 3.7.16, 3.7.17, 3.7.18, 3.7.19, 3.7.2, 3.7.20, 3.7.21, 3.7.22, 3.7.23, 3.7.24, 3.7.25, 3.7.26, 3.7.27, 3.7.28, 3.7.29, 3.7.3, 3.7.30, 3.7.31, 3.7.32, 3.7.33, 3.7.34, 3.7.35, 3.7.36, 3.7.37, 3.7.38, 3.7.4, 3.7.5, 3.7.6, 3.7.7, 3.7.8, 3.7.9].

  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, reverting ./composer.json to its original content.
user@user:/opt/lampp/htdocs/projects/paypal/ipn$ 

I have also tried: composer install which throws the following error:

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

  Problem 1
    - The requested PHP extension ext-curl * is missing from your system. Install or enable PHP's curl extension.
  Problem 2
    - phpunit/phpunit 3.7.9 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.8 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.7 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.6 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.5 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.4 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.38 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.37 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.36 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.35 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.34 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.33 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.32 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.31 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.30 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.3 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.29 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.28 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.27 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.26 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.25 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.24 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.23 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.22 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.21 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.20 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.2 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.19 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.18 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.17 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.16 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.15 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.14 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.13 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.12 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.11 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.10 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 3.7.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - Installation request for phpunit/phpunit 3.7.* -> satisfiable by phpunit/phpunit[3.7.0, 3.7.1, 3.7.10, 3.7.11, 3.7.12, 3.7.13, 3.7.14, 3.7.15, 3.7.16, 3.7.17, 3.7.18, 3.7.19, 3.7.2, 3.7.20, 3.7.21, 3.7.22, 3.7.23, 3.7.24, 3.7.25, 3.7.26, 3.7.27, 3.7.28, 3.7.29, 3.7.3, 3.7.30, 3.7.31, 3.7.32, 3.7.33, 3.7.34, 3.7.35, 3.7.36, 3.7.37, 3.7.38, 3.7.4, 3.7.5, 3.7.6, 3.7.7, 3.7.8, 3.7.9].

  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.
user@user:/opt/lampp/htdocs/projects/paypal/ipn$ 

What could I be doing wrong?

Thank you all in advance.

Here's composer.json of my application:

{
    "require" : {
        "paypal/rest-api-sdk-php" : "*"
    }
}

Answer 1

A platform requirement indicated in the composer.json of paypal/rest-api-sdk-php cannot be fulfilled, that is why composer refuses to install the package:

Problem 1
    - The requested PHP extension ext-curl * is missing from your system. Install or enable PHP's curl extension.

Requiring a package with --ignore-platform-reqs

If all you care about is requiring the package, you can run

$ composer require paypal/rest-api-sdk-php --ignore-platform-reqs

For reference, see https://getcomposer.org/doc/03-cli.md#require :

--ignore-platform-reqs : ignore php , hhvm , lib-* and ext-* requirements and force the installation even if the local machine does not fulfill these. See also the platform config option.

Installing packages with --ignore-platform-reqs

Assuming the package

• has already been added to composer.json
• composer.lock is up-to-date with the changes in composer.json

you can run

$ composer install --ignore-platform-reqs

For reference, see https://getcomposer.org/doc/03-cli.md#install :

--ignore-platform-reqs : ignore php , hhvm , lib-* and ext-* requirements and force the installation even if the local machine does not fulfill these. See also the platform config option.

To find out whether composer.lock is up-to-date with the changes in composer.json , run

$ composer validate

Note : You may still not be able to use the package, as the package depends on ext/curl to be installed. To make sure you can install and use the package on a machine, you should install ext/curl on that machine.