Composer: clarify that the PHP libxml extension is required

The `simplexml` and `xmlwriter` extensions both need the `libxml` extension, so the requirement not being explicit was not problematic, as it was already enforced via the other two requirements.

Still, better to be explicit.

Includes adding the extension to the requirements check and the related tests.

Author: jrfnl

composer.json

@@ -35,6 +35,7 @@
     },
     "require": {
         "php": ">=7.2.0",
+        "ext-libxml": "*",
         "ext-simplexml": "*",
         "ext-tokenizer": "*",
         "ext-xmlwriter": "*"

requirements.php

@@ -41,6 +41,7 @@ function checkRequirements()
 
     $requiredExtensions = array(
         'tokenizer',
+        'libxml',
         'xmlwriter',
         'SimpleXML',
     );

scripts/BuildRequirementsCheckMatrix.php

@@ -82,7 +82,7 @@ public function getBuilds()
      */
     private function getValidBuilds()
     {
-        $extensions = ['minimal' => 'none, tokenizer, xmlwriter, SimpleXML'];
+        $extensions = ['minimal' => 'none, tokenizer, libxml, xmlwriter, SimpleXML'];
 
         $builds = [];
         foreach ($this->validPhp as $php) {
@@ -145,10 +145,10 @@ private function getInvalidPHPBuilds()
     private function getMissingExtensionsBuilds()
     {
         $extensions = [
-            'missing tokenizer'     => 'none, xmlwriter, SimpleXML',
-            'missing xmlwriter'     => ':xmlwriter',
-            'missing SimpleXML'     => ':SimpleXML',
-            'missing both XML exts' => 'none, tokenizer',
+            'missing tokenizer'    => 'none, xmlwriter, SimpleXML',
+            'missing xmlwriter'    => ':xmlwriter',
+            'missing SimpleXML'    => ':SimpleXML',
+            'missing all XML exts' => 'none, tokenizer',
         ];
 
         $builds = [];