PHP Manual . ... PHP 3 <= 3.0.18, PHP 4 >= 4.0.0) strtolower -- Make a string lowercase . string strtolower ( string str) . Returns string with all alphabetic characters converted to lowercase. ... This means that in i.e. the default "C" locale, characters such as umlaut-A (д) will not be converted. Example 1. strtolower() example . str = "Mary Had A Little Lamb and She LOVED It So"; $str = strtolower($str); print $str; # Prints mary had a little lamb and she loved it so . ...