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Rule ``explicit_string_variable``
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Converts implicit variables into explicit ones in double-quoted strings or
heredoc syntax.

Description
-----------

The reasoning behind this rule is the following:
- When there are two valid ways of doing the same thing, using both is
confusing, there should be a coding standard to follow.
- PHP manual marks ``"$var"`` syntax as implicit and ``"{$var}"`` syntax as
explicit: explicit code should always be preferred.
- Explicit syntax allows word concatenation inside strings, e.g.
``"{$var}IsAVar"``, implicit doesn't.
- Explicit syntax is easier to detect for IDE/editors and therefore has
colors/highlight with higher contrast, which is easier to read.
Backtick operator is skipped because it is harder to handle; you can use
``backtick_to_shell_exec`` fixer to normalize backticks to strings.

Examples
--------

Example #1
~~~~~~~~~~

.. code-block:: diff

   --- Original
   +++ New
    <?php
   -$a = "My name is $name !";
   -$b = "I live in $state->country !";
   -$c = "I have $farm[0] chickens !";
   +$a = "My name is {$name} !";
   +$b = "I live in {$state->country} !";
   +$c = "I have {$farm[0]} chickens !";

Rule sets
---------

The rule is part of the following rule set:

- `@PhpCsFixer <./../../ruleSets/PhpCsFixer.rst>`_

References
----------

- Fixer class: `PhpCsFixer\\Fixer\\StringNotation\\ExplicitStringVariableFixer <./../../../src/Fixer/StringNotation/ExplicitStringVariableFixer.php>`_
- Test class: `PhpCsFixer\\Tests\\Fixer\\StringNotation\\ExplicitStringVariableFixerTest <./../../../tests/Fixer/StringNotation/ExplicitStringVariableFixerTest.php>`_

The test class defines officially supported behaviour. Each test case is a part of our backward compatibility promise.