title: Composer installing: the json extension is missing
body: <p>I wrote in Linux Terminal: 
  <code>curl -s https://getcomposer.org/installer | php</code>
  It said: </p>
  
  <pre><code> #!/usr/bin/env php
  Some settings on your machine make Composer unable to work properly.
  Make sure that you fix the issues listed below and run this script again:
  
  The json extension is missing.
  Install it or recompile php without --disable-json
  </code></pre>
  
  <p>I did this:  <code>apt-get install php5-json</code>
  So, I think, it's installed. But the strange thing is, when I write <code>php -m</code>
  It gives me a list without json:</p>
  
  <pre><code>[PHP Modules]
  bcmath
  bz2
  calendar
  Core
  ctype
  date
  dba
  dom
  ereg
  exif
  fileinfo
  filter
  ftp
  gettext
  hash
  iconv
  libxml
  mbstring
  mhash
  openssl
  pcntl
  pcre
  Phar
  posix
  Reflection
  session
  shmop
  SimpleXML
  soap
  sockets
  SPL
  standard
  sysvmsg
  sysvsem
  sysvshm
  tokenizer
  wddx
  xml
  xmlreader
  xmlwriter
  zip
  zlib
  
  [Zend Modules]
  </code></pre>
  
  <p>And when I write  <code>php5-fpm -m</code>
  It sends this:</p>
  
  <pre><code>[PHP Modules]
  bcmath
  bz2
  calendar
  cgi-fcgi
  Core
  ctype
  curl
  date
  dba
  dom
  ereg
  exif
  fileinfo
  filter
  ftp
  gettext
  hash
  iconv
  json
  libxml
  mbstring
  mcrypt
  mhash
  mysqli
  openssl
  pcre
  PDO
  pdo_mysql
  Phar
  posix
  Reflection
  session
  shmop
  SimpleXML
  soap
  sockets
  SPL
  standard
  sysvmsg
  sysvsem
  sysvshm
  tokenizer
  wddx
  xml
  xmlreader
  xmlwriter
  zip
  zlib
  
  [Zend Modules]
  </code></pre>
  
  <p>What am I doing wrong? I haven't found the solution at GitHub and Google</p>