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LIII. Multi-Byte String Functions

Introduction

There are many languages in which all characters can be expressed by single byte. Multi-byte character codes are used to express many characters for many languages. mbstring is developed to handle Japanese characters. However, many mbstring functions are able to handle character encoding other than Japanese.

A multi-byte character encoding represents single character with consecutive bytes. Some character encoding has shift(escape) sequences to start/end multi-byte character strings. Therefore, a multi-byte character string may be destroyed when it is divided and/or counted unless multi-byte character encoding safe method is used. This module provides multi-byte character safe string functions and other utility functions such as conversion functions.

Since PHP is basically designed for ISO-8859-1, some multi-byte character encoding does not work well with PHP. Therefore, it is important to set mbstring.internal_encoding to a character encoding that works with PHP.

PHP4 Character Encoding Requirements

  • Per byte encoding

  • Single byte characters in range of 00h-7fh which is compatible with ASCII

  • Multi-byte characters without 00h-7fh

These are examples of internal character encoding that works with PHP and does NOT work with PHP. 

Character encodings work with PHP: 
ISO-8859-*, EUC-JP, UTF-8

Character encodings do NOT work with PHP:
JIS, SJIS

Character encoding, that does not work with PHP, may be converted with mbstring's HTTP input/output conversion feature/function.

Note: SJIS should not be used for internal encoding unless the reader is familiar with parser/compiler, character encoding and character encoding issues.

Note: If you use database with PHP, it is recommended that you use the same character encoding for both database and internal encoding for ease of use and better performance.

If you are using PostgreSQL, it supports character encoding that is different from backend character encoding. See the PostgreSQL manual for details.

How to Enable mbstring

mbstring is an extended module. You must enable module with configure script. Refer to the Install section for details.

The following configure options are related to mbstring module.

  • --enable-mbstring : Enable mbstring functions. This option is required to use mbstring functions.

  • --enable-mbstr-enc-trans : Enable HTTP input character encoding conversion using mbstring conversion engine. If this feature is enabled, HTTP input character encoding may be converted to mbstring.internal_encoding automatically.

HTTP Input and Output

HTTP input/output character encoding conversion may convert binary data also. Users are supposed to control character encoding conversion if binary data is used for HTTP input/output.

If enctype for HTML form is set to multipart/form-data, mbstring does not convert character encoding in POST data. If it is the case, strings are needed to be converted to internal character encoding.

  • HTTP Input

    There is no way to control HTTP input character conversion from PHP script. To disable HTTP input character conversion, it has to be done in php.ini.

    Example 1. Disable HTTP input conversion in php.ini 

    ;; Disable HTTP Input conversion
mbstring.http_input = pass

    When using PHP as an Apache module, it is possible to override PHP ini setting per Virtual Host in httpd.conf or per directory with .htaccess. Refer to the Configuration section and Apache Manual for details.

  • HTTP Output

    There are several ways to enable output character encoding conversion. One is using php.ini, another is using ob_start() with mb_output_handler() as ob_start callback function.

    Note: For PHP3-i18n users, mbstring's output conversion differs from PHP3-i18n. Character encoding is converted using output buffer.

Example 2. php.ini setting example

;; Enable output character encoding conversion for all PHP pages

;; Enable Output Buffering
output_buffering    = On

;; Set mb_output_handler to enable output conversion
output_handler      = mb_output_handler

Example 3. Script example

<?php

// Enable output character encoding conversion only for this page

// Set HTTP output character encoding to SJIS
mb_http_output('SJIS');

// Start buffering and specify "mb_output_handler" as
// callback function
ob_start('mb_output_handler');

?>

Supported Character Encoding

Currently, the following character encoding is supported by mbstring module. Caracter encoding may be specified for mbstring functions' encoding parameter.

The following character encoding is supported in this PHP extension :

UCS-4, UCS-4BE, UCS-4LE, UCS-2, UCS-2BE, UCS-2LE, UTF-32, UTF-32BE, UTF-32LE, UCS-2LE, UTF-16, UTF-16BE, UTF-16LE, UTF-8, UTF-7, ASCII, EUC-JP, SJIS, eucJP-win, SJIS-win, ISO-2022-JP, JIS, ISO-8859-1, ISO-8859-2, ISO-8859-3, ISO-8859-4, ISO-8859-5, ISO-8859-6, ISO-8859-7, ISO-8859-8, ISO-8859-9, ISO-8859-10, ISO-8859-13, ISO-8859-14, ISO-8859-15, byte2be, byte2le, byte4be, byte4le, BASE64, 7bit, 8bit and UTF7-IMAP.

php.ini entry, which accepts encoding name, accepts "auto" and "pass" also. mbstring functions, which accepts encoding name, and accepts "auto".

If "pass" is set, no character encoding conversion is performed.

If "auto" is set, it is expanded to "ASCII,JIS,UTF-8,EUC-JP,SJIS".

See also mb_detect_order()

Note: "Supported character encoding" does not mean that it works as internal character code.

php.ini settings

  • mbstring.internal_encoding defines default internal character encoding.

  • mbstring.http_input defines default HTTP input character encoding.

  • mbstring.http_output defines default HTTP output character encoding.

  • mbstring.detect_order defines default character code detection order. See also mb_detect_order().

  • mbstring.substitute_character defines character to substitute for invalid character encoding.

Web Browsers are supposed to use the same character encoding when submitting form. However, browsers may not use the same character encoding. See mb_http_input() to detect character encoding used by browsers.

If enctype is set to multipart/form-data in HTML forms, mbstring does not convert character encoding in POST data. The user must convert them in the script, if conversion is needed.

Although, browsers are smart enough to detect character encoding in HTML. charset is better to be set in HTTP header. Change default_charset according to character encoding.

Example 4. php.ini setting example

;; Set default internal encoding
;; Note: Make sure to use character encoding works with PHP
mbstring.internal_encoding    = UTF-8  ; Set internal encoding to UTF-8

;; Set default HTTP input character encoding
;; Note: Script cannot change http_input setting.
mbstring.http_input           = pass    ; No conversion. 
mbstring.http_input           = auto    ; Set HTTP input to auto
                                ; "auto" is expanded to "ASCII,JIS,UTF-8,EUC-JP,SJIS"
mbstring.http_input           = SJIS    ; Set HTTP2 input to  SJIS
mbstring.http_input           = UTF-8,SJIS,EUC-JP ; Specify order

;; Set default HTTP output character encoding 
mbstring.http_output          = pass    ; No conversion
mbstring.http_output          = UTF-8   ; Set HTTP output encoding to UTF-8

;; Set default character encoding detection order
mbstring.detect_order         = auto    ; Set detect order to auto
mbstring.detect_order         = ASCII,JIS,UTF-8,SJIS,EUC-JP ; Specify order

;; Set default substitute character
mbstring.substitute_character = 12307   ; Specify Unicode value
mbstring.substitute_character = none    ; Do not print character
mbstring.substitute_character = long    ; Long Example: U+3000,JIS+7E7E

Example 5. php.ini setting for EUC-JP users

;; Disable Output Buffering
output_buffering      = Off

;; Set HTTP header charset
default_charset       = EUC-JP    

;; Set HTTP input encoding conversion to auto
mbstring.http_input   = auto 

;; Convert HTTP output to EUC-JP
mbstring.http_output  = EUC-JP    

;; Set internal encoding to EUC-JP
mbstring.internal_encoding = EUC-JP    

;; Do not print invalid characters
mbstring.substitute_character = none

Example 6. php.ini setting for SJIS users

;; Enable Output Buffering
output_buffering     = On

;; Set mb_output_handler to enable output conversion
output_handler       = mb_output_handler

;; Set HTTP header charset
default_charset      = Shift_JIS

;; Set http input encoding conversion to auto
mbstring.http_input  = auto 

;; Convert to SJIS
mbstring.http_output = SJIS    

;; Set internal encoding to EUC-JP
mbstring.internal_encoding = EUC-JP    

;; Do not print invalid characters
mbstring.substitute_character = none

Overload of PHP string functions by mbstring functions with multibyte support

Because almost PHP application written for language using single-byte character encoding, there are some difficulties for multibyte string handling including japanese. Almost PHP string functions such as substr() do not support multibyte string.

Multibyte extension (mbstring) has some PHP string functions with multibyte support (ex. substr() supports mb_substr()).

Multibyte extension (mbstring) also supports 'function overloading' to add multibyte string functionality without code modification. Using function overloading, some PHP string functions will be oveloaded multibyte string functions. For example, mb_substr() is called instead of substr() if function overloading is enabled. Function overload makes easy to port application supporting only single-byte encoding for multibyte application.

mbstring.func_overload in php.ini should be set some positive value to use function overloading. The value should specify the category of overloading functions, sbould be set 1 to enable mail function overloading. 2 to enable string functions, 4 to regular expression functions. For example, if is set for 7, mail, strings, regex functions should be overloaded. The list of overloaded functions are shown in below.

Table 1. Functions to be overloaded

value of mbstring.func_overloadoriginal functionoverloaded function
1mail()mb_send_mail()
2strlen()mb_strlen()
2strpos()mb_strpos()
2strrpos()mb_strrpos()
2substr()mb_substr()
4ereg()mb_ereg()
4eregi()mb_eregi()
4ereg_replace()mb_ereg_replace()
4eregi_replace()mb_eregi_replace()
4split()mb_split()

Basics for Japanese multi-byte character

Most Japanese characters need more than 1 byte per character. In addition, several character encoding schemas are used under a Japanese environment. There are EUC-JP, Shift_JIS(SJIS) and ISO-2022-JP(JIS) character encoding. As Unicode becomes popular, UTF-8 is used also. To develop Web applications for a Japanese environment, it is important to use the character set for the task in hand, whether HTTP input/output, RDBMS and E-mail.

  • Storage for a character can be up to six bytes

  • A multi-byte character is usually twice of the width compared to single-byte characters. Wider characters are called "zen-kaku" - meaning full width, narrower characters are called "han-kaku" - meaning half width. "zen-kaku" characters are usually fixed width.

  • Some character encoding defines shift(escape) sequence for entering/exiting multi-byte character strings.

  • ISO-2022-JP must be used for SMTP/NNTP.

  • "i-mode" web site is supposed to use SJIS.

References

Multi-byte character encoding and its related issues are very complex. It is impossible to cover in sufficient detail here. Please refer to the following URLs and other resources for further readings.

  • Unicode/UTF/UCS/etc

    http://www.unicode.org/

  • Japanese/Korean/Chinese character information

    ftp://ftp.ora.com/pub/examples/nutshell/ujip/doc/cjk.inf

Table of Contents
mb_convert_encoding -- Convert character encoding
mb_convert_kana --  Convert "kana" one from another ("zen-kaku" ,"han-kaku" and more)
mb_convert_variables -- Convert character code in variable(s)
mb_decode_mimeheader -- Decode string in MIME header field
mb_decode_numericentity --  Decode HTML numeric string reference to character
mb_detect_encoding -- Detect character encoding
mb_detect_order --  Set/Get character encoding detection order
mb_encode_mimeheader -- Encode string for MIME header
mb_encode_numericentity --  Encode character to HTML numeric string reference
mb_ereg -- Regular expression match with multibyte support
mb_ereg_match --  Regular expression match for multibyte string
mb_ereg_replace -- Replace regular expression with multibyte support
mb_ereg_search --  Multibyte regular expression match for predefined multibyte string
mb_ereg_search_getpos --  Returns start point for next regular expression match
mb_ereg_search_getregs --  Retrive the result from the last multibyte regular expression match
mb_ereg_search_init --  Setup string and regular expression for multibyte regular expression match
mb_ereg_search_pos --  Return position and length of matched part of multibyte regular expression for predefined multibyte string
mb_ereg_search_regs --  Returns the matched part of multibyte regular expression
mb_ereg_search_setpos --  Set start point of next regular expression match
mb_eregi --  Regular expression match ignoring case with multibyte support
mb_eregi_replace --  Replace regular expression with multibyte support ignoring case
mb_get_info -- Get internal settings of mbstring
mb_http_input -- Detect HTTP input character encoding
mb_http_output -- Set/Get HTTP output character encoding
mb_internal_encoding --  Set/Get internal character encoding
mb_language --  Set/Get current language
mb_output_handler --  Callback function converts character encoding in output buffer
mb_parse_str --  Parse GET/POST/COOKIE data and set global variable
mb_preferred_mime_name -- Get MIME charset string
mb_regex_encoding --  Returns current encoding for multibyte regex as string
mb_send_mail --  Send encoded mail.
mb_split -- Split multibyte string using regular expression
mb_strcut -- Get part of string
mb_strimwidth -- Get truncated string with specified width
mb_strlen -- Get string length
mb_strpos --  Find position of first occurrence of string in a string
mb_strrpos --  Find position of last occurrence of a string in a string
mb_strwidth -- Return width of string
mb_substitute_character -- Set/Get substitution character
mb_substr -- Get part of string
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