feature #35625 [String] Add the s() helper method (fancyweb)

This PR was merged into the 5.1-dev branch.

Discussion
----------

[String] Add the s() helper method

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | https://github.com/symfony/symfony/pull/35578#discussion_r374512887
| License       | MIT
| Doc PR        | -

This method will be useful in our code base, and to anyone that doesn't really understand the differences between UnicodeString and ByteString.

Commits
-------

659cdf1871 [String] Add the s() helper method

src/Symfony/Component/String/CHANGELOG.md

@@ -8,6 +8,8 @@ CHANGELOG
  * made `AbstractString::width()` follow POSIX.1-2001
  * added `LazyString` which provides memoizing stringable objects
  * The component is not marked as `@experimental` anymore.
+ * Added the `s()` helper method to get either an `UnicodeString` or `ByteString` instance,
+   depending of the input string UTF-8 compliancy.
 
 5.0.0
 -----

src/Symfony/Component/String/Resources/functions.php

@@ -20,3 +20,11 @@ function b(string $string = ''): ByteString
 {
     return new ByteString($string);
 }
+
+/**
+ * @return UnicodeString|ByteString
+ */
+function s(string $string): AbstractString
+{
+    return preg_match('//u', $string) ? new UnicodeString($string) : new ByteString($string);
+}

src/Symfony/Component/String/Tests/FunctionsTest.php

@@ -0,0 +1,39 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\String\Tests;
+
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\String\AbstractString;
+use Symfony\Component\String\ByteString;
+use function Symfony\Component\String\s;
+use Symfony\Component\String\UnicodeString;
+
+final class FunctionsTest extends TestCase
+{
+    /**
+     * @dataProvider provideS
+     */
+    public function testS(AbstractString $expected, string $input)
+    {
+        $this->assertEquals($expected, s($input));
+    }
+
+    public function provideS()
+    {
+        return [
+            [new UnicodeString('foo'), 'foo'],
+            [new UnicodeString('अनुच्छेद'), 'अनुच्छेद'],
+            [new ByteString("b\x80ar"), "b\x80ar"],
+            [new ByteString("\xfe\xff"), "\xfe\xff"],
+        ];
+    }
+}