title: Composer-php install error
body: <p>I have a composer.json with content:</p>
  
  <pre><code>{
    "require":{
      "paypal/rest-api-sdk-php": "*"
    }
  }
  </code></pre>
  
  <p>On trying to install the composer by below command <code>composer install</code>,</p>
  
  <p>I'm getting this error message:</p>
  
  <blockquote>
    <p>Your requirements could not be resolved to an installable set of packages.</p>
  </blockquote>
  
  <p>The complete error log:</p>
  
  <pre><code>Problem 1
  - paypal/rest-api-sdk-php v1.7.1 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.7.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.6.4 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.6.3 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.6.2 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.6.1 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.6.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.5.1 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.5.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.4.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.3.2 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.3.1 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.3.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.2.1 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.2.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.1.1 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.1.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v1.0.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.9.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.8.6 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.8.5 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.8.4 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.8.3 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.8.2 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.8.1 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.8.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.7.1 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.7.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.6.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.16.1 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.16.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.15.1 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.15.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.14.2 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.14.1 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.14.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.13.2 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.13.1 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.13.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.12.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.11.1 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.11.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php v0.10.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php V0.5.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php 1.9.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php 1.8.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php 1.7.4 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php 1.7.3 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php 1.7.2 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php 1.11.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - paypal/rest-api-sdk-php 1.10.0 requires ext-curl * -&gt; the requested PHP extension curl is missing from your system.
  - Installation request for paypal/rest-api-sdk-php * -&gt; satisfiable by paypal/rest-api-sdk-php[1.10.0, 1.11.0, 1.7.2, 1.7.3, 1.7.4, 1.8.0, 1.9.0, V0.5.0, v0.10.0, v0.11.0, v0.11.1, v0.12.0, v0.13.0, v0.13.1, v0.13.2, v0.14.0, v0.14.1, v0.14.2, v0.15.0, v0.15.1, v0.16.0, v0.16.1, v0.6.0, v0.7.0, v0.7.1, v0.8.0, v0.8.1, v0.8.2, v0.8.3, v0.8.4, v0.8.5, v0.8.6, v0.9.0, v1.0.0, v1.1.0, v1.1.1, v1.2.0, v1.2.1, v1.3.0, v1.3.1, v1.3.2, v1.4.0, v1.5.0, v1.5.1, v1.6.0, v1.6.1, v1.6.2, v1.6.3, v1.6.4, v1.7.0, v1.7.1].
  
  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.
  </code></pre>
  
  <p>I'm new to this. What am I missing here?</p>