Category is a named constant (or string) specifying the category of the functions affected by the locale setting:
LC_ALL for all of the below
LC_COLLATE for string comparison, see strcoll()
LC_CTYPE for character classification and conversion, for example strtoupper()
LC_MONETARY for localeconv()
LC_NUMERIC for decimal separator (See also localeconv())
LC_TIME for date and time formatting with strftime()
If locale is the empty string "", the locale names will be set from the values of environment variables with the same names as the above categories, or from "LANG".
If locale is NULL or "0", the locale setting is not affected, only the current setting is returned.
If locale is an array or followed by additional parameters then each array element or parameter is tried to be set as new locale until success. This is useful if a locale is known under different names on different systems or for providing a fallback for a possibly not available locale.
Note: Passing multiple locales is not available before PHP 4.3.0
Setlocale returns the new current locale, or FALSE if the locale functionality is not implemented on your platform, the specified locale does not exist or the category name is invalid. An invalid category name also causes a warning message. Category/locale names can be found in RFC 1766 and ISO 639.
Note: The return value of setlocale() depends on the system that PHP is running. It returns exactly what the system setlocale function returns.
Tip: Windows users will find useful information about locale strings at Microsoft's MSDNwebsite. Supported language strings can be found at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vclib/html/_crt_language_strings.asp and supported country/region strings at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vclib/html/_crt_country_strings.asp. Windows systems support the three letter codes for country/region specified by ISO 3166-Alpha-3, which can be found at this Unicode website .
Example 1. setlocale() Examples
Example 2. setlocale() Examples for Windows