From 4e46d362a4fce130299fd0c1a7890a4760c113e2 Mon Sep 17 00:00:00 2001
From: Louis-Arnaud Catoire <la.catoire@gmail.com>
Date: Tue, 25 Aug 2026 22:04:16 +0200
Subject: [PATCH] errorfunc: document the error handler stack

set_error_handler() saves the handler that was active until then on an
internal stack and installs the new one in its place;
restore_error_handler() pops the saved one back, together with the
error_levels mask it was registered with. Neither page said so, which
invites restoring the previous handler by passing the return value of
set_error_handler() back to it. That grows the stack instead of
shrinking it, and resets error_levels to E_ALL.

Adds a caution against that pattern and an example calling the previous
handler from the new one. The seealso simplelists are unwrapped from
their para, matching the exception handler pages.
---
 .../functions/restore-error-handler.xml       | 32 +++---
 .../errorfunc/functions/set-error-handler.xml | 99 +++++++++++++++----
 2 files changed, 100 insertions(+), 31 deletions(-)

diff --git a/reference/errorfunc/functions/restore-error-handler.xml b/reference/errorfunc/functions/restore-error-handler.xml
index 31cbc6c532bb..a374b5de3820 100644
--- a/reference/errorfunc/functions/restore-error-handler.xml
+++ b/reference/errorfunc/functions/restore-error-handler.xml
@@ -12,11 +12,17 @@
    <type>true</type><methodname>restore_error_handler</methodname>
    <void/>
   </methodsynopsis>
-  <para>
-   Used after changing the error handler function using
-   <function>set_error_handler</function>, to revert to the previous error
-   handler (which could be the built-in or a user defined function). 
-  </para>
+  <simpara>
+   Pops the error handler that was active before the most recent call to
+   <function>set_error_handler</function> off the internal stack of error
+   handlers and makes it active again, together with the
+   <parameter>error_levels</parameter> mask it was registered with.
+   The restored handler could be the built-in or a user defined function.
+  </simpara>
+  <simpara>
+   Calling this function more often than <function>set_error_handler</function>
+   leaves the built-in error handler active; no error is raised.
+  </simpara>
  </refsect1>
 
  <refsect1 role="parameters">
@@ -67,15 +73,13 @@ Invalid serialized value.
 
  <refsect1 role="seealso">
   &reftitle.seealso;
-  <para>
-   <simplelist>
-    <member><function>error_reporting</function></member>
-    <member><function>set_error_handler</function></member>
-    <member><function>get_error_handler</function></member>
-    <member><function>restore_exception_handler</function></member>
-    <member><function>trigger_error</function></member>
-   </simplelist>
-  </para>
+  <simplelist>
+   <member><function>error_reporting</function></member>
+   <member><function>set_error_handler</function></member>
+   <member><function>get_error_handler</function></member>
+   <member><function>restore_exception_handler</function></member>
+   <member><function>trigger_error</function></member>
+  </simplelist>
  </refsect1>
 </refentry>
 <!-- Keep this comment at the end of the file
diff --git a/reference/errorfunc/functions/set-error-handler.xml b/reference/errorfunc/functions/set-error-handler.xml
index f425fd317229..aa620d26cc20 100644
--- a/reference/errorfunc/functions/set-error-handler.xml
+++ b/reference/errorfunc/functions/set-error-handler.xml
@@ -13,10 +13,13 @@
    <methodparam><type class="union"><type>callable</type><type>null</type></type><parameter>callback</parameter></methodparam>
    <methodparam choice="opt"><type>int</type><parameter>error_levels</parameter><initializer><constant>E_ALL</constant></initializer></methodparam>
   </methodsynopsis>
-  <para>
-   Sets a user function (<parameter>callback</parameter>) to handle
-   errors in a script.
-  </para>
+  <simpara>
+   Sets a user function (<parameter>callback</parameter>) to handle errors in
+   a script.
+   The handler that was active until then is saved on an internal stack of
+   error handlers, from which <function>restore_error_handler</function> pops
+   it back.
+  </simpara>
   <para>
    This function can be used to define custom error handlers during runtime,
    for example in applications which need to do file/data cleanup when a critical
@@ -62,10 +65,13 @@
     <varlistentry>
      <term><parameter>callback</parameter></term>
      <listitem>
-      <para>
-       If &null; is passed, the handler is reset to its default state.
+      <simpara>
+       If &null; is passed, no user-defined handler is left active and errors
+       fall back to the built-in error handler.
+       The handler that was active until then is still saved on the stack and
+       can be brought back with <function>restore_error_handler</function>.
        Otherwise, the handler is a callback with the following signature:
-      </para>
+      </simpara>
       <para>
        <methodsynopsis>
         <type>bool</type><methodname><replaceable>handler</replaceable></methodname>
@@ -329,24 +335,83 @@ vector d - fatal error
 <b>My ERROR</b> [256] log(x) for x <= 0 is undefined, you used: scale = -2.5<br />
   Fatal error on line 35 in file trigger_error.php, PHP 5.2.1 (FreeBSD)<br />
 Aborting...<br />
+]]>
+    </screen>
+   </example>
+  </para>
+  <para>
+   <example>
+    <title>Calling the previous handler from the new one</title>
+    <simpara>
+     The return value of <function>set_error_handler</function> is the handler
+     that was active until then. Calling it from the new handler keeps the
+     existing behaviour while adding to it. This only works if that handler is
+     a user defined function; &null; is returned when the built-in one was
+     active.
+    </simpara>
+    <programlisting role="php">
+<![CDATA[
+<?php
+function logging_handler(int $errno, string $errstr): bool
+{
+    echo "logged: $errstr\n";
+    return true;
+}
+
+set_error_handler('logging_handler');
+
+$previous = set_error_handler(function (int $errno, string $errstr) use (&$previous): bool {
+    echo "counted: $errstr\n";
+    return $previous !== null ? $previous($errno, $errstr) : false;
+});
+
+trigger_error("something happened", E_USER_WARNING);
+
+restore_error_handler();
+trigger_error("only logged now", E_USER_WARNING);
+
+restore_error_handler();
+?>
+]]>
+    </programlisting>
+    &example.outputs;
+    <screen>
+<![CDATA[
+counted: something happened
+logged: something happened
+logged: only logged now
 ]]>
     </screen>
    </example>
   </para>
  </refsect1>
 
+ <refsect1 role="notes">
+  &reftitle.notes;
+  <caution>
+   <simpara>
+    Do not revert to the previous handler by passing the return value of
+    <function>set_error_handler</function> back to it.
+    Doing so pushes a further entry onto the stack instead of removing one,
+    so the stack grows without bound in loops and long-running processes, and
+    <parameter>error_levels</parameter> is reset to <constant>E_ALL</constant>,
+    silently widening the range of errors the restored handler receives.
+    <function>restore_error_handler</function> is the intended way to revert
+    to the previous handler.
+   </simpara>
+  </caution>
+ </refsect1>
+
  <refsect1 role="seealso">
   &reftitle.seealso;
-  <para>
-   <simplelist>
-    <member><classname>ErrorException</classname></member>
-    <member><function>error_reporting</function></member>
-    <member><function>restore_error_handler</function></member>
-    <member><function>get_error_handler</function></member>
-    <member><function>trigger_error</function></member>
-    <member><link linkend="errorfunc.constants">error level constants</link></member>
-   </simplelist>
-  </para>
+  <simplelist>
+   <member><exceptionname>ErrorException</exceptionname></member>
+   <member><function>error_reporting</function></member>
+   <member><function>restore_error_handler</function></member>
+   <member><function>get_error_handler</function></member>
+   <member><function>trigger_error</function></member>
+   <member><link linkend="errorfunc.constants">error level constants</link></member>
+  </simplelist>
  </refsect1>
 </refentry>
 <!-- Keep this comment at the end of the file