Ahora estoy tratando de usar composer para instalar mi paquete php pero recibí este mensaje de error: $ composer install

Loading composer repositories with package information
Installing dependencies (including require-dev)
  - Installing filp/whoops (1.0.7)
    Downloading: 100%          

[Symfony\Component\Process\Exception\RuntimeException]                                   
The Process class relies on proc_open, which is not available on your PHP installation. 

editar:

después de habilitar la función proc_open, recibí este error y también configuré mi php memory_limit a 384M:

Fatal error: Uncaught exception 'ErrorException' with message 'proc_open(): fork failed - Cannot allocate memory' in phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php:990
Stack trace:
#0 [internal function]: Composer\Util\ErrorHandler::handle(2, 'proc_open(): fo...', 'phar:///usr/loc...', 990, Array)
#1 phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php(990): proc_open('stty -a | grep ...', Array, NULL, NULL, NULL, Array)
#2 phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php(832): Symfony\Component\Console\Application->getSttyColumns()
#3 phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php(797): Symfony\Component\Console\Application->getTerminalDimensions()
#4 phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php(731): Symfony\Component\Console\Application->getTerminalWidth()
#5 phar:///usr/local/bin/com in phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php on line 990