restore_error_handler

Isolde is kind of wrong. The error handlers are stacked with set_error_handler(), and popped with restore_error_handler(). Here i put an example:
<?php
 mysql_connect("inexistent"); //Generate an error. The actual error handler is set by default
 function foo1() {echo "<br>Error foo1<br>";}
 function foo2() {echo "<br>Error foo2<br>";}
 function foo3() {echo "<br>Error foo3<br>";}
 
 set_error_handler("foo1"); //current error handler: foo1
 set_error_handler("foo2"); //current error handler: foo2
 set_error_handler("foo3"); //current error handler: foo3
 
 mysql_connect("inexistent"); 
 restore_error_handler(); //now, current error handler: foo2
 mysql_connect("inexistent"); 
 restore_error_handler(); //now, current error handler: foo1
 mysql_connect("inexistent"); 
 restore_error_handler(); //now current error handler: default handler
 mysql_connect("inexistent");
 restore_error_handler(); //now current error handler: default handler (The stack can't pop more)
?>

Calling restore_error_handler from within an error handler might result in unexpected behaviour:
<?php
error_reporting(0);
set_error_handler('handleError1');
trigger_error('1-stack:h1');
set_error_handler('handleError2');
trigger_error('2-stack:h1,h2');
trigger_error('6-stack:h1,h2');
trigger_error('7-stack:h1,h2');
function handleError1($code, $message, $file = '', $line = 0, $context = array())
{
 echo __METHOD__ . ' ' . $message . PHP_EOL;
}
function handleError2($code, $message, $file = '', $line = 0, $context = array())
{
 trigger_error('3-DEFAULT'); // This will use the php's default error handler
 echo __METHOD__ . ' ' . $message . PHP_EOL;
 set_error_handler('handleError3');
 trigger_error('4-stack:h1,h2,h3');
 restore_error_handler(); // This will restore the handleError1 instead of the default error handler
 trigger_error('5-DEFAULT');
}
function handleError3($code, $message, $file = '', $line = 0, $context = array())
{
 echo __METHOD__ . ' ' . $message . PHP_EOL;
}
?>

The above code will output:
handleError1 1-stack:h1
handleError2 2-stack:h1,h2
handleError3 4-stack:h1,h2,h3
handleError1 5-DEFAULT
handleError1 6-stack:h1,h2
handleError1 7-stack:h1,h2
The following workaround can be used:
<?php
error_reporting(0);
set_error_handler('handleError1');
trigger_error('1-stack:h1');
set_error_handler('handleError2');
trigger_error('2-stack:h1,h2');
trigger_error('6-stack:h1,h2');
trigger_error('7-stack:h1,h2');
function handleError1($code, $message, $file = '', $line = 0, $context = array())
{
 echo __METHOD__ . ' ' . $message . PHP_EOL;
}
function handleError2($code, $message, $file = '', $line = 0, $context = [])
{
 restore_error_handler(); // This will restore the previous error handler
 set_error_handler('count', 0); // Set a dummy method for error handling, it will never be called because $error_type = 0
 try
 {
 trigger_error('3-DEFAULT');
 echo __METHOD__ . ' ' . $message . PHP_EOL;
 set_error_handler('handleError3');
 trigger_error('4-stack:h1,h2,h3');
 restore_error_handler();
 trigger_error('5-DEFAULT');
 }
 finally
 {
 restore_error_handler(); // Restore the previous error handler
 set_error_handler('handleError2'); // Set the current error handler again
 }
}
function handleError3($code, $message, $file = '', $line = 0, $context = [])
{
 echo __METHOD__ . ' ' . $message . PHP_EOL;
}
?>

which will output:
handleError1 1-stack:h1
handleError2 2-stack:h1,h2
handleError3 4-stack:h1,h2,h3
handleError2 6-stack:h1,h2
handleError3 4-stack:h1,h2,h3
handleError2 7-stack:h1,h2
handleError3 4-stack:h1,h2,h3

As the docs say, restore_error_handler() revert to the *previous error handler*... even if it is the same. A bug made me set twice my custom error handler and later when I was calling restore_error_handler() to restore the built-in handler nothing seemed to happen... this puzzled me for a while!

Works also for restoring nested error handlers:
<?php
error_reporting(E_ALL);
echo '<pre>';
set_error_handler(function($errno, $errstr, $errfile, $errline, array $errcontext) {
 echo 'ErrorHandler 1: ' , $errstr , PHP_EOL;
});
trigger_error('Error 1');
set_error_handler(function($errno, $errstr, $errfile, $errline, array $errcontext) {
 echo 'ErrorHandler 2: ' , $errstr , PHP_EOL;
});
trigger_error('Error 2');
restore_error_handler();
trigger_error('Error 3');
restore_error_handler();
trigger_error('Error 4');
?>