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)
?>

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!

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

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');

?>