xml_get_current_byte_index()

The xml_get_current_byte_index() function is a PHP built-in function that returns how many bytes of the XML document the parser has already consumed. It is part of the legacy XML Parser extension, which parses documents in a streaming, SAX-style way: instead of loading the whole document into a tree, it fires callbacks as it walks through the markup. This function tells you where the parser is when one of those callbacks runs.

You normally call it from inside a handler registered with xml_set_element_handler() or xml_set_character_data_handler(). It is most useful for reporting parsing progress on a large file, or for locating where in the source a particular element or piece of content appeared — for example, to build an error message that points at a byte offset.

This page covers the function's syntax, return value, two practical examples, and the gotchas to watch for.

Syntax

xml_get_current_byte_index(XMLParser $parser): int

Parameters

• $parser — the parser handle returned by xml_parser_create() or xml_parser_create_ns(). On PHP 8.0+ this is an XMLParser object; on PHP 7.x it is a resource.

Return value

Returns the byte offset (a zero-based integer) of the parser's current position in the document. Because it counts bytes, not characters, a multibyte UTF-8 file will report a larger index than its character count. If you need a line or column instead, use the companion functions xml_get_current_line_number() and xml_get_current_column_number().

The value is only meaningful while parsing is in progress (i.e. inside a handler). Calling it before xml_parse() has started returns 0.

Usage Examples

Let's take a look at some practical examples of using xml_get_current_byte_index() in PHP.

Example 1: Retrieving the Current Byte Index of an XML Parser

Suppose you have an XML file "data.xml" that you want to parse using the XML Parser extension in PHP. You can use the xml_get_current_byte_index() function inside a SAX element handler to retrieve the current byte index when an element starts, like this:

Retrieving the Current Byte Index of an XML Parser in PHP

$parser = xml_parser_create();
xml_parser_set_option($parser, XML_OPTION_CASE_FOLDING, 0);

function start_handler($parser, $name, $attrs) {
    $byte_index = xml_get_current_byte_index($parser);
    echo "Element <$name> starts at byte index: $byte_index\n";
}

xml_set_element_handler($parser, "start_handler", null);

$xml_data = file_get_contents("data.xml");
xml_parse($parser, $xml_data, true);
xml_parser_free($parser);

This code creates an XML parser using xml_parser_create(), and sets an option to turn off case folding. It defines a start_handler callback that calls xml_get_current_byte_index() to get the parser's position whenever an element begins. The handler is registered with xml_set_element_handler(), and xml_parse() processes the file. Finally, it frees the memory used by the XML parser using xml_parser_free().

Example 2: Tracking Parsing Progress

Suppose you have a large XML file and want to display a progress indicator while parsing it. You can use the XML Parser extension with a character data handler to track the byte index as the parser reads through the file, like this:

Tracking Parsing Progress in PHP

$parser = xml_parser_create();
xml_parser_set_option($parser, XML_OPTION_CASE_FOLDING, 0);

$total_bytes = filesize("data.xml");
$last_reported = 0;

function progress_handler($parser, $data) {
    global $last_reported, $total_bytes;
    $current = xml_get_current_byte_index($parser);
    if ($current - $last_reported > 1024) { // Report every 1KB
        $progress = round(($current / $total_bytes) * 100);
        echo "Parsing progress: $progress%\n";
        $last_reported = $current;
    }
}

xml_set_character_data_handler($parser, "progress_handler");
xml_parse($parser, file_get_contents("data.xml"), true);
xml_parser_free($parser);

This code creates an XML parser and calculates the total file size. It defines a progress_handler callback that checks the current byte index against the last reported position. If more than 1KB has been read, it calculates and prints the parsing percentage. The handler is registered with xml_set_character_data_handler(), and xml_parse() processes the file. Finally, it frees the memory used by the XML parser using xml_parser_free().

Notes and gotchas

• Bytes, not characters. On a UTF-8 document with multibyte characters, the returned offset can be larger than the character position. Don't treat it as a character count.
• Call it inside a handler. The byte index only reflects a real position while a callback is running. Outside parsing it returns 0.
• The offset can point slightly past the markup. Depending on the libexpat buffering, the reported index may sit at the end of the token that triggered the callback rather than at its exact start. Use it for approximate progress and locating, not byte-exact slicing.
• This is the legacy SAX API. For most new code, the tree-based SimpleXML or DOM extensions are easier. Reach for the XML Parser extension when you specifically need streaming for very large files. See the overview of PHP XML parsers to choose.

Conclusion

In this article, we've discussed PHP's xml_get_current_byte_index() function and how it can be used to retrieve the current byte index of an XML parser during SAX-style parsing. We've explained what the function does, its syntax, and provided examples of how it can be used in practical scenarios. By following these examples, you can easily track the parser's position and use it in your PHP applications to monitor progress or locate specific elements.

Practice