The `setlocale()` function in PHP is used to set locale information, which affects how certain functions handle language, monetary, time, and other region-specific settings.PHP 8.PHP 8.1,PHP 8.2,PHP 8.3 and PHP 8.4.

Basic Syntax<?phpsetlocale(int $category, string $locales, string ...$rest): string|false?>

Parameters

`$category`: One of the predefined locale constants:`LC_ALL` - All of the below`LC_COLLATE` - For string comparison (e.g., `strcoll()`)`LC_CTYPE` - For character classification and conversion`LC_MONETARY` - For localeconv()`LC_NUMERIC` - For decimal separator`LC_TIME` - For date and time formatting with `strftime()` (deprecated in PHP 8.1)`LC_MESSAGES` - For system responses (if supported by OS)`$locales`: A string or array of strings naming the locale(s) to try

Example Usage in PHP 8.2<?php// Set locale for all categories to US English$locale_set = setlocale(LC_ALL, 'en_US.utf8', 'en_US', 'english');if ($locale_set === false) {echo "Locale not supported on this system.\n";} else {echo "Locale set to: " . $locale_set . "\n";}// Format a number according to locale$number = 1234.56;echo "Formatted number: " . number_format($number, 2) . "\n";// Output depends on locale - in US: 1,234.56// Set just the monetary locale to German (Germany)setlocale(LC_MONETARY, 'de_DE.utf8', 'de_DE', 'deu_deu');$monetary_info = localeconv();echo "German currency symbol: " . $monetary_info['currency_symbol'] . "\n";// Output: ?>

Important Notes for PHP 8.21. Deprecation Notice**: The `strftime()` function (which used locale settings) is deprecated in PHP 8.1. In PHP 8.2, you should use `IntlDateFormatter` or `date()` instead for date formatting.2. Locale Availability**: The locales you can use depend on what's installed on your server. You can check available locales with:```bashlocale -a3. Best Practice**: Always check the return value of `setlocale()` as it returns `false` if the locale isn't available.4. UTF-8 Recommendation**: When possible, use UTF-8 locales (like 'en_US.utf8' instead of just 'en_US') for better Unicode support.5. Thread Safety**: Locale settings may affect the entire process in some SAPI environments, not just the current script.