Write the syntax of php mb_strtolower function?

PHP mb_strtolower function is used to converts uppercase character into lowercase in the string.

$str1 = "I am good ";
$str1 = mb_strtolower($str1);
echo $str1; 

mb_strtolower function does not affect the Unicode characters also.

$str = "Τάχιστη ηξ βαφής ψημένη γη, δρασκελίζει κυνός";
$str = mb_strtolower($str, 'UTF-8');
echo $str;