Wiki/Requirements: improve information about required PHP extensions

wiki/Requirements.md

@@ -1,19 +1,41 @@
-PHP_CodeSniffer 4.x requires PHP 7.2.0 or greater.  
+PHP_CodeSniffer 4.x requires **PHP 7.2.0 or greater**.  
 _Prior to PHP_CodeSniffer 4.0.0, the minimum PHP requirement was 5.4.0._
 
 Additionally, PHP_CodeSniffer requires the following PHP extensions to be enabled:
-- Tokenizer: used by the core tokenizer to process PHP files
-- SimpleXML: used to process ruleset XML files
-- XMLWriter: used to create some report formats
+- **[Tokenizer][tokenizer]**: used by the core tokenizer to process PHP files
+- **[SimpleXML][simplexml]**: used to process ruleset XML files
+- **[XMLWriter][xmlwriter]**: used to create some report formats
+- **[libxml]**: required by the SimpleXML and XMLWriter extensions
 
-The following PHP extensions are not required, but are strongly recommended:
-- iconv: used for accurate character length calculation in files containing multibyte characters. Without this extension, some sniffs, like `Generic.Files.LineLength`, may report incorrect results for lines containing non-ASCII characters, as PHP_CodeSniffer will fall back to byte-based length calculations.
-- PCNTL: required for parallel processing via the `--parallel` CLI option. Without this extension, PHP_CodeSniffer will not be able to check multiple files simultaneously.
+The following PHP extensions are not required, but are **_strongly recommended_**:
+- **[DOM][dom]**: used for displaying the sniff documentation via the `--generator=...` option.
+- **[iconv]**: used for accurate character length calculation in files containing multibyte characters.  
+    Without this extension, some sniffs, like `Generic.Files.LineLength`, may report incorrect results for lines containing non-ASCII characters, as PHP_CodeSniffer will fall back to byte-based length calculations.  
+    Additionally, the following reports will not work without this extension if the project `encoding` is not UTF-8: [Checkstyle][wiki-report-checkstyle], [JUnit][wiki-report-junit] and [XML][wiki-report-xml].
+- **[json]**: required for the cache functionality, as well as for the [JSON report][wiki-report-json] output.
+- **[PCNTL][pcntl]**: required for parallel processing via the `--parallel` CLI option.  
+    Without this extension, PHP_CodeSniffer will not be able to check multiple files simultaneously.
 
-Individual sniffs may have additional requirements such as external applications and scripts. See the [Configuration Options](https://github.com/PHPCSStandards/PHP_CodeSniffer/wiki/Configuration-Options) manual page for a list of these requirements.
+Individual sniffs may have additional requirements such as external applications and scripts. See the [Configuration Options][wiki-config] manual page for a list of these requirements.
 
 > [!WARNING]
-> The gRPC PHP extension is known to cause PHP_CodeSniffer to hang when running with parallel processing enabled. If the gRPC extension is loaded, either do not use the `--parallel` CLI option or configure the extension's ini settings as follows:
+> The **[gRPC PHP extension][grpc]** is known to cause PHP_CodeSniffer to hang when running with parallel processing enabled. If the gRPC extension is loaded, either do not use the `--parallel` CLI option or configure the extension's ini settings as follows:
 > ```text
 > phpcs -d grpc.enable_fork_support=1 -d grpc.poll_strategy=epoll1 [other options] <file|directory>
 > ```
+
+[dom]:       https://www.php.net/book.dom
+[grpc]:      https://grpc.io/docs/languages/php/
+[iconv]:     https://www.php.net/book.iconv
+[json]:      https://www.php.net/book.json
+[libxml]:    https://www.php.net/book.libxml
+[pcntl]:     https://www.php.net/book.pcntl
+[simplexml]: https://www.php.net/book.simplexml
+[tokenizer]: https://www.php.net/book.tokenizer
+[xmlwriter]: https://www.php.net/book.xmlwriter
+
+[wiki-config]:            https://github.com/PHPCSStandards/PHP_CodeSniffer/wiki/Configuration-Options
+[wiki-report-checkstyle]: https://github.com/PHPCSStandards/PHP_CodeSniffer/wiki/Reporting#printing-a-checkstyle-report
+[wiki-report-json]:       https://github.com/PHPCSStandards/PHP_CodeSniffer/wiki/Reporting#printing-a-json-report
+[wiki-report-junit]:      https://github.com/PHPCSStandards/PHP_CodeSniffer/wiki/Reporting#printing-a-junit-report
+[wiki-report-xml]:        https://github.com/PHPCSStandards/PHP_CodeSniffer/wiki/Reporting#printing-an-xml-report