Document fallback value syntax for ini variables (closes #4191) (#4192)

Author: jimwins

install/ini.xml

@@ -88,20 +88,23 @@
     PHP to attempt to read &php.ini; from the root filesystem if it exists.
    </para>
   </note>
-  <para>
-   Using environment variables can be used in &php.ini; as shown below.
-  </para>
-  <para>
-   <example>
-    <title>&php.ini; Environment Variables</title>
-     <programlisting role="ini">
+  <simpara>
+   Environment variables can be referenced within configuration values
+   in &php.ini; as shown below. As of PHP 8.3.0, a fallback value can
+   be specified that will be used when the referenced variable is not
+   defined.
+  </simpara>
+  <example>
+   <title>&php.ini; Environment Variables</title>
+    <programlisting role="ini">
 <![CDATA[
 ; PHP_MEMORY_LIMIT is taken from environment
 memory_limit = ${PHP_MEMORY_LIMIT}
+; If PHP_MAX_EXECUTION_TIME is not defined, it will fall back to 30
+max_execution_time = ${PHP_MAX_EXECUTION_TIME:-30}
 ]]>
-     </programlisting>
-    </example>
-  </para>
+   </programlisting>
+  </example>
   <para>
    The &php.ini; directives handled by extensions are documented
    on the respective pages of the extensions themselves. A <link linkend="ini">list of