strtolower

strtolower is a built-in function in PHP that converts all alphabetic characters in a string to their lowercase equivalents. The transformation is locale-aware, meaning it follows the current locale’s rules for casing as defined by the runtime environment.

Syntax and behavior: string strtolower(string $string). The function returns the input string with ASCII letters converted to lowercase, and it may handle non-ASCII characters according to the active locale. Because it depends on locale settings, results can vary between environments or configurations set with setlocale or LC_CTYPE.

Caveats and alternatives: For UTF-8 and other multibyte encodings, strtolower may not reliably convert non-Latin characters. In such cases, it is common to use mb_strtolower($string, 'UTF-8'), which applies multibyte-aware casing suitable for Unicode. Regardless of approach, strtolower preserves the string length and does not remove or add characters.

Examples and usage considerations: strtolower('Hello World!') yields 'hello world!'. Developers should choose between strtolower and multibyte alternatives based on the character set of the input data and the desired locale behavior. In contexts requiring consistent Unicode handling, mb_strtolower is generally preferred.