Supported Character Encodings
PHP Character Encoding Requirements
Multibyte String
PHP Manual

# Function Overloading Feature

You might often find it difficult to get an existing PHP application to work in a given multibyte environment. This happens because most PHP applications out there are written with the standard string functions such as substr(), which are known to not properly handle multibyte-encoded strings.

mbstring supports a 'function overloading' feature which enables you to add multibyte awareness to such an application without code modification by overloading multibyte counterparts on the standard string functions. For example, mb_substr() is called instead of substr() if function overloading is enabled. This feature makes it easy to port applications that only support single-byte encodings to a multibyte environment in many cases.

To use function overloading, set mbstring.func_overload in php.ini to a positive value that represents a combination of bitmasks specifying the categories of functions to be overloaded. It should be set to 1 to overload the mail() function. 2 for string functions, 4 for regular expression functions. For example, if it is set to 7, mail, strings and regular expression functions will be overloaded. The list of overloaded functions are shown below.

Functions to be overloaded
value of mbstring.func_overload | original function | overloaded function
--- | --- | ---
1 | mail() | mb_send_mail()
2 | strlen() | mb_strlen()
2 | strpos() | mb_strpos()
2 | strrpos() | mb_strrpos()
2 | substr() | mb_substr()
2 | strtolower() | mb_strtolower()
2 | strtoupper() | mb_strtoupper()
2 | substr_count() | mb_substr_count()
4 | ereg() | mb_ereg()
4 | eregi() | mb_eregi()
4 | ereg_replace() | mb_ereg_replace()
4 | eregi_replace() | mb_eregi_replace()
4 | split() | mb_split()

> Note: It is not recommended to use the function overloading option in the per-directory context, because it's not confirmed yet to be stable enough in a production environment and may lead to undefined behaviour.

Supported Character Encodings
PHP Character Encoding Requirements
Multibyte String
PHP Manual