minor symfony#12157 Simplified the docs about Symfony technical requirements (javiereguiluz)

This PR was squashed before being merged into the 4.3 branch (closes symfony#12157).

Discussion
----------

Simplified the docs about Symfony technical requirements

The `symfony` binary is so great that we can simplify lots of things thanks to the new `check:requirements` command.

@nicolas-grekas quick question. Is this still true?

```
Install PHP 7.1 or higher and these PHP extensions (which are installed and
enabled by default in most PHP 7 installations): `Ctype`_, `iconv`_, `JSON`_,
`PCRE`_, `Session`_, `SimpleXML`_, and `Tokenizer`_;
```

Do we require all those extensions or things like `ctype` are no longer required because of some polyfills? Thanks.

Commits
-------

455d613 Simplified the docs about Symfony technical requirements

Author: OskarStark

_build/redirection_map

@@ -444,3 +444,4 @@
 /routing/extra_information /routing
 /console/request_context /routing
 /form/action_method /forms
+/reference/requirements /setup

best_practices/creating-the-project.rst

@@ -39,10 +39,9 @@ create files and execute the following commands:
 This command creates a new directory called ``blog`` that contains a fresh new
 project based on the most recent stable Symfony version available.
 
-.. tip::
+.. seealso::
 
-    The technical requirements to run Symfony are simple. If you want to check
-    if your system meets those requirements, read :doc:`/reference/requirements`.
+    Check out the :ref:`technical requirements for running Symfony applications <symfony-tech-requirements>`.
 
 Structuring the Application
 ---------------------------

deployment.rst

@@ -118,8 +118,8 @@ you'll need to do:
 A) Check Requirements
 ~~~~~~~~~~~~~~~~~~~~~
 
-Use the :doc:`Symfony Requirements Checker </reference/requirements>` to check
-if your server meets the technical requirements to run Symfony applications.
+Use the ``check:requirements`` command to check if your server meets the
+:ref:`technical requirements for running Symfony applications <symfony-tech-requirements>`.
 
 .. _b-configure-your-app-config-parameters-yml-file:
 

migration.rst

@@ -62,9 +62,9 @@ First of all your environment needs to be able to support the minimum
 requirements for both applications. In other words, when the Symfony
 release you aim to use requires PHP 7.1 and your existing application
 does not yet support this PHP version, you will probably have to upgrade
-your legacy project. You can find out the
-:doc:`requirements </reference/requirements>` for running Symfony and
-compare them with your current application's environment to make sure you
+your legacy project. Use the ``check:requirements`` command to check if your
+server meets the :ref:`technical requirements for running Symfony applications <symfony-tech-requirements>`
+and compare them with your current application's environment to make sure you
 are able to run both applications on the same system. Having a test
 system, that is as close to the production environment as possible,
 where you can just install a new Symfony project next to the existing one

reference/index.rst

@@ -22,6 +22,5 @@ Reference Documents
 
     dic_tags
     events
-    requirements
 
 .. include:: /reference/map.rst.inc

reference/map.rst.inc

@@ -28,4 +28,3 @@
 
   * :doc:`/reference/dic_tags`
   * :doc:`/reference/events`
-  * :doc:`/reference/requirements`

reference/requirements.rst

@@ -10,17 +10,27 @@ Installing & Setting up the Symfony Framework
     Do you prefer video tutorials? Check out the `Stellar Development with Symfony`_
     screencast series.
 
+.. _symfony-tech-requirements:
+
 Technical Requirements
 ----------------------
 
 Before creating your first Symfony application you must:
 
-* Make sure to have PHP 7.1 or higher installed (and :doc:`these PHP extensions </reference/requirements>`
-  which are installed and enabled by default by PHP);
+* Install PHP 7.1 or higher and these PHP extensions (which are installed and
+  enabled by default in most PHP 7 installations): `Ctype`_, `iconv`_, `JSON`_,
+  `PCRE`_, `Session`_, `SimpleXML`_, and `Tokenizer`_;
 * `Install Composer`_, which is used to install PHP packages;
 * `Install Symfony`_, which creates in your computer a binary called ``symfony``
   that provides all the tools you need to develop your application locally.
 
+The ``symfony`` binary provides a tool to check if your computer meets these
+requirements. Open your console terminal and run this command:
+
+.. code-block:: terminal
+
+    $ symfony check:requirements
+
 .. _creating-symfony-applications:
 
 Creating Symfony Applications
@@ -57,6 +67,12 @@ will create a new ``my_project_name/`` directory, download some dependencies
 into it and even generate the basic directories and files you'll need to get
 started. In other words, your new application is ready!
 
+.. note::
+
+    The project's cache and logs directory (by default, ``<project>/var/cache/``
+    and ``<project>/var/log/``) must be writable by the web server. If you have
+    any issue, read how to :doc:`set up permissions for Symfony applications </setup/file_permissions>`.
+
 Running Symfony Applications
 ----------------------------
 
@@ -81,12 +97,6 @@ Open your browser and navigate to ``http://localhost:8000/``. If everything is
 working, you'll see a welcome page. Later, when you are finished working, stop
 the server by pressing ``Ctrl+C`` from your terminal.
 
-.. tip::
-
-    If you're having any problems running Symfony, your system may be missing
-    some technical requirements. Use the :doc:`Symfony Requirements Checker </reference/requirements>`
-    tool to make sure your system is set up.
-
 .. _install-existing-app:
 
 Setting up an Existing Symfony Project
@@ -258,3 +268,10 @@ Learn More
 .. _`Main recipe repository`: https://github.com/symfony/recipes
 .. _`Contrib recipe repository`: https://github.com/symfony/recipes-contrib
 .. _`Symfony Recipes documentation`: https://github.com/symfony/recipes/blob/master/README.rst
+.. _`iconv`: https://php.net/book.iconv
+.. _`JSON`: https://php.net/book.json
+.. _`Session`: https://php.net/book.session
+.. _`Ctype`: https://php.net/book.ctype
+.. _`Tokenizer`: https://php.net/book.tokenizer
+.. _`SimpleXML`: https://php.net/book.simplexml
+.. _`PCRE`: https://php.net/book.pcre