title: PHP 7, Symfony 3: Fatal error 1 abstract method and must therefore be declared abstract or implement the remaining methods
body: <p>After a php update from 5.6 to 7, Symfony 3 raise this exception:</p>
  
  <blockquote>
    <p>Fatal error: Class
    Symfony\Component\HttpFoundation\Session\Storage\Proxy\SessionHandlerProxy
    contains 1 abstract method and must therefore be declared abstract or
    implement the remaining methods (SessionHandlerInterface::write) in
    \vendor\symfony\symfony\src\Symfony\Component\HttpFoundation\Session\Storage\Proxy\SessionHandlerProxy.php
    on line 19</p>
  </blockquote>
  
  <p>is Symfony 3 incompatible with PHP 7? same code work on php 5.6</p>
  
  <p>composer show -i output:</p>
  
  <pre><code>doctrine/annotations                 v1.2.7  Docblock Annotations Parser
  doctrine/cache                       v1.6.0  Caching library offering an o...
  doctrine/collections                 v1.3.0  Collections Abstraction library
  doctrine/common                      v2.6.1  Common Library for Doctrine p...
  doctrine/dbal                        v2.5.3  Database Abstraction Layer
  doctrine/doctrine-bundle             1.6.1   Symfony DoctrineBundle
  doctrine/doctrine-cache-bundle       1.2.2   Symfony Bundle for Doctrine C...
  doctrine/inflector                   v1.1.0  Common String Manipulations w...
  doctrine/instantiator                1.0.5   A small, lightweight utility ...
  doctrine/lexer                       v1.0.1  Base library for a lexer that...
  doctrine/orm                         v2.5.3  Object-Relational-Mapper for PHP
  incenteev/composer-parameter-handler v2.1.2  Composer script handling your...
  jdorn/sql-formatter                  v1.2.17 a PHP SQL highlighting library
  kriswallsmith/assetic                v1.3.2  Asset Management for PHP
  monolog/monolog                      1.17.2  Sends your logs to files, soc...
  paragonie/random_compat              v1.2.2  PHP 5.x polyfill for random_b...
  psr/log                              1.0.0   Common interface for logging ...
  sensio/distribution-bundle           v5.0.3  Base bundle for Symfony Distr...
  sensio/framework-extra-bundle        v3.0.12 This bundle provides a way to...
  sensio/generator-bundle              v3.0.3  This bundle generates code fo...
  sensiolabs/security-checker          v3.0.2  A security checker for your c...
  swiftmailer/swiftmailer              v5.4.1  Swiftmailer, free feature-ric...
  symfony/assetic-bundle               v2.7.1  Integrates Assetic into Symfony2
  symfony/monolog-bundle               v2.8.2  Symfony MonologBundle
  symfony/phpunit-bridge               v2.8.1  Symfony PHPUnit Bridge
  symfony/polyfill-intl-icu            v1.1.1  Symfony polyfill for intl's I...
  symfony/polyfill-mbstring            v1.1.1  Symfony polyfill for the Mbst...
  symfony/polyfill-php56               v1.1.1  Symfony polyfill backporting ...
  symfony/polyfill-php70               v1.1.1  Symfony polyfill backporting ...
  symfony/polyfill-util                v1.1.1  Symfony utilities for portabi...
  symfony/swiftmailer-bundle           v2.3.9  Symfony SwiftmailerBundle
  symfony/symfony                      v3.0.3  The Symfony PHP framework
  twig/twig                            v1.24.0 Twig, the flexible, fast, and...
  </code></pre>
  
  <p>php.ini</p>
  
  <pre><code>[opcache]
  opcache.enable=1
  opcache.memory_consumption=128
  opcache.interned_strings_buffer=16
  opcache.max_accelerated_files=6000
  opcache.use_cwd=1
  opcache.validate_timestamps=1
  opcache.save_comments=1
  opcache.fast_shutdown=1
  </code></pre>