Add history for PHP 6, 7, and 8 (#3358) (#3576)

appendices/history.xml

@@ -202,6 +202,66 @@
     of millions of domains around the world.
    </para>
   </sect2>
+
+  <sect2 xml:id="history.php6">
+   <title>PHP 6</title>
+   <para>
+    The plan for PHP 6 was to feature deep support for Unicode in
+    the engine and language. This work was later abandoned, but other
+    features that had been targeted for PHP 6 were instead incorporated
+    into PHP 5.3 (<link linkend="language.namespaces">namespaces</link>)
+    and PHP 5.4 (<link linkend="language.oop5.traits">trait</link>,
+    "short" array syntax).
+   </para>
+  </sect2>
+
+  <sect2 xml:id="history.php7">
+   <title>PHP 7</title>
+   <para>
+    PHP 7.0 was released in 2015, and featured another major version of
+    the core, <literal>Zend Engine 3.0</literal>, which included
+    significant performance improvements (up to twice as fast as PHP
+    5.6) and memory usage, consistent 64-bit support, improvements
+    to exceptions, a secure random number generator, the
+    <link linkend="language.operators.comparison.coalesce">null coalescing
+    operator (??)</link>, <link linkend="language.oop5.anonymous">anonymous
+    classes</link>, and other features.
+   </para>
+   <para>
+    Later releases of the PHP 7 series introduced more language features
+    such as the <link linkend="function.list">short list syntax</link> (7.1),
+    object parameter and return type declaration (7.2), a new flexible
+    <link linkend="language.types.string.syntax.heredoc">heredoc</link>
+    and <link linkend="language.types.string.syntax.nowdoc">nowdoc</link>
+    syntax (7.3), typed properties (7.4), and a new
+    <link linkend="class.ffi">foreign function interface</link> (7.4).
+   </para>
+  </sect2>
+
+  <sect2 xml:id="history.php8">
+   <title>PHP 8</title>
+   <para>
+    First released in 2020, PHP 8.0 was another major update of the PHP
+    language. It contained many new features and optimizations including
+    <link linkend="functions.named-arguments">named arguments</link>,
+    <link linkend="language.types.declarations.composite.union">union
+    types</link>, <link linkend="language.attributes">attributes</link>,
+    <link linkend="language.oop5.decon.constructor.promotion">constructor
+    property promotion</link>,
+    <link linkend="control-structures.match"><literal>match</literal></link>
+    expressions, the <link linkend="language.oop5.basic.nullsafe">nullsafe
+    operator (<literal>?-></literal>)</link>, a new optimizing JIT compiler,
+    and improvements in the type system, error handling, and consistency.
+   </para>
+   <para>
+    Later releases of the PHP 8 series introduced new language features
+    including <link linkend="language.enumerations">enumerations</link> (8.1),
+    <link linkend="language.fibers">fibers</link> (8.1),
+    <link linkend="language.oop5.basic.class.readonly">readonly classes</link> (8.2),
+    Disjunctive Normal Form (DNF) Types (8.2), and typed class
+    constants (8.3).
+   </para>
+  </sect2>
  </sect1>
  
  <sect1 xml:id="history.php.related">