PHP WTF? Error handler is not called, when display_errors=On

tst.php

<?php
/**
 * WTF? Error handler is not called, when display_errors=On
 */


function _errorHandler($severity, $message, $file, $line, $context) {
	$GLOBALS['err'] = func_get_args();
//	echo "Error ($severity) $message in $file:$line\n";
}
set_error_handler('_errorHandler');


// display_errors = Off, works fine
{
	ini_set('display_errors', 0);


	// Calls error handler, fine
	$err = NULL;
	$tmp = substr(new stdClass(), 1, 1);
	var_dump($err[0], $err[1]);


	// Calls error handler, fine
	$err = NULL;
	$tmp = json_encode(array("bad utf\xFF"));
	var_dump($err[0], $err[1]);
}



// display_errors = On, WTF???
{
	ini_set('display_errors', 1);


	// Calls error handler, fine
	$err = NULL;
	$tmp = substr(new stdClass(), 1, 1);
	var_dump($err[0], $err[1]);


	// Does NOT call error handler, WTF???
	$err = NULL;
	$tmp = json_encode(array("bad utf\xFF"));
	var_dump($err[0], $err[1]);
}