Please provide a short but complete example script.

Here as requested is an example which works fine under 4.2.2, and causes an endless loop in 4.3.2:


<!-- BEGIN XML FILE:  shapes.xml -->

<SCREEN>
  <INFO>This will render a random surprise shape</INFO>
  <RANDOM shapes="SQUARE TRIANGLE CIRCLE"/>
</SCREEN>

<!-- END XML FILE -->


### CUT HERE ###


<!-- BEGIN PHP FILE:  shapes.php -->

<?php
$file = "shapes.xml";

function startElement($parser, $name, $attribs) {
    switch ($name)
    {
        case "RANDOM":
            $list = explode(" ", $attribs["SHAPES"]);
            $num = count($list);
            $rnd = rand(1, $num) - 1;
            $xml = "<" . $list[$rnd] . "/>";
            if ( !xml_parse($parser, $xml) )
            {
                print xml_error_string(xml_get_error_code($parser));
            }
            break;
        case "SQUARE":
            print "\n       ################\n";
            print "       ################\n";
            print "       ################\n";
            print "       ################\n";
            print "       ################\n";
            print "       ################\n";
            print "       ################\n";
            print "       ################\n";
            break;
        case "TRIANGLE":
            print "\n              ##       \n";
            print "             ####      \n";
            print "            ######     \n";
            print "           ########    \n";
            print "          ##########   \n";
            print "         ############  \n";
            print "        ############## \n";
            print "       ################\n";
            break;
        case "CIRCLE":
            print "\n           ########    \n";
            print "         ############  \n";
            print "        ############## \n";
            print "        ############## \n";
            print "        ############## \n";
            print "        ############## \n";
            print "         ############  \n";
            print "           ########    \n";
            break;
    }
}
function endElement($parser, $name) {
}

function characterData($parser, $data) {
    print "<b>$data</b>";
}

function defaultHandler($parser, $data) {
    if (substr($data, 0, 1) == "&" && substr($data, -1, 1) == ";") {
        printf('<font color="#aa00aa">%s</font>',
            htmlspecialchars($data));
    } else {
        printf('<font size="-1">%s</font>',
            htmlspecialchars($data));
    }
}

function new_xml_parser($file) {
    global $parser_file;

    $xml_parser = xml_parser_create();
    xml_parser_set_option($xml_parser, XML_OPTION_CASE_FOLDING, 1);
    xml_set_element_handler($xml_parser, "startElement", "endElement");
    xml_set_character_data_handler($xml_parser, "characterData");
    xml_set_default_handler($xml_parser, "defaultHandler");

    if (!($fp = @fopen($file, "r"))) {
        return false;
    }
    if (!is_array($parser_file)) {
        settype($parser_file, "array");
    }
    $parser_file[$xml_parser] = $file;
    return array($xml_parser, $fp);
}

if (!(list($xml_parser, $fp) = new_xml_parser($file))) {
    die("could not open XML input");
}

print "<pre>";
while ($data = fread($fp, 4096)) {
    if (!xml_parse($xml_parser, $data, feof($fp))) {
        die(sprintf("XML error: %s at line %d\n",
            xml_error_string(xml_get_error_code($xml_parser)),
            xml_get_current_line_number($xml_parser)));
    }
}
print "</pre>";
print "parse complete\n";
xml_parser_free($xml_parser);

?>

<!-- END PHP FILE -->

Works fine with PHP 4.2.3, breaks with 4.3.1, 4.3.2, 4.3.3-dev.

It also crashes:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 1024 (runnable)]
0x40678cd9 in __strtod_internal (nptr=0x8ad63ec "SCREEN", endptr=0xbfe0225c, group=0) at strtod.c:419
(gdb) bt
#0  0x40678cd9 in __strtod_internal (nptr=0x8ad63ec "SCREEN", endptr=0xbfe0225c, group=0) at strtod.c:419
#1  0x4067dc59 in strtod (nptr=0x8ad63ec "SCREEN", endptr=0xbfe0225c) at strtod.c:1425
#2  0x82c5345 in is_numeric_string (str=0x8ad63ec "SCREEN", length=6, lval=0xbfe022c8, dval=0xbfe022bc, 
    allow_errors=0 '\000') at /usr/src/web/php/php4/Zend/zend_operators.h:94
#3  0x82c4ebe in zendi_smart_strcmp (result=0xbfe0242c, s1=0x8ad63ac, s2=0x88a7764)
    at /usr/src/web/php/php4/Zend/zend_operators.c:1670
#4  0x82c3736 in compare_function (result=0xbfe0242c, op1=0x8ad63ac, op2=0x88a7764)
    at /usr/src/web/php/php4/Zend/zend_operators.c:1137
#5  0x82c41a6 in is_equal_function (result=0xbfe0242c, op1=0x8ad63ac, op2=0x88a7764)
    at /usr/src/web/php/php4/Zend/zend_operators.c:1285
#6  0x82dc798 in execute (op_array=0x88a60c8) at /usr/src/web/php/php4/Zend/zend_execute.c:1931
#7  0x82bc741 in call_user_function_ex (function_table=0x85a7cb0, object_pp=0x0, function_name=0x88a1744, 
    retval_ptr_ptr=0xbfe02c44, param_count=3, params=0x8ad6554, no_separation=1, symbol_table=0x0)
    at /usr/src/web/php/php4/Zend/zend_execute_API.c:566
#8  0x82bbee7 in call_user_function (function_table=0x85a7cb0, object_pp=0x88a0a58, function_name=0x88a1744, 
    retval_ptr=0x8ad6514, param_count=3, params=0xbfe02cdc) at /usr/src/web/php/php4/Zend/zend_execute_API.c:408
#9  0x8261550 in xml_call_handler (parser=0x88a0a1c, handler=0x88a1744, argc=3, argv=0xbfe02cdc)
    at /usr/src/web/php/php4/ext/xml/xml.c:377
#10 0x826207c in _xml_startElementHandler (userData=0x88a0a1c, name=0x8ad6326 "SCREEN", attributes=0x88a0d08)
    at /usr/src/web/php/php4/ext/xml/xml.c:661

Diff betweeb 4.2.3 and 4.3.3-dev ext/xml doesn't give any significant changes, so it must be something else that has changed and just hasn't been changed also in ext/xml, call_user_function() maybe?

xml_parse cant be used like this as you already found out: http://mail.libexpat.org/pipermail/expat-discuss/2003-June/001039.html

True, but that doesn't explain why it worked with all versions of PHP prior to 4.3.X, and then stopped working ;)

We've worked around this "issue", but it has definately added a little overhead to the script.

I still think the XML parser should be more robust and dynamic.

If it worked then you were lucky. Running with the bundled expat library, the behavior I see when testing is consistent in 4.1.2, 4.2.2 and 4.3.x (infinite loop), which is why they said not to do that and that it wont work.

As far as the parser robustness and dynamic capaibilities, you have to take it up with the expat developers, but I think they already gave you their answer to that one.