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.
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/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.
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.
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.
Note: For PHP3-i18n users, mbstring's output conversion differs from PHP3-i18n. Character encoding is converted using output buffer.
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.
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
Example 5. php.ini setting for EUC-JP users
Example 6. php.ini setting for SJIS users
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) 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_overload||original function||overloaded function|
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.
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.
Japanese/Korean/Chinese character information