DELIMITER $$
 
DROP FUNCTION IF EXISTS `replace_ci`$$
CREATE FUNCTION `replace_ci` ( str TEXT,needle CHAR(255),str_rep CHAR(255))
    RETURNS TEXT
    DETERMINISTIC
    BEGIN
        DECLARE return_str TEXT DEFAULT '';
        DECLARE lower_str TEXT;
        DECLARE lower_needle TEXT;
        DECLARE pos INT DEFAULT 1;
        DECLARE old_pos INT DEFAULT 1;
 
        SELECT LOWER(str) INTO lower_str;
        SELECT LOWER(needle) INTO lower_needle;
        SELECT locate(lower_needle, lower_str, pos) INTO pos;
        WHILE pos > 0 DO
            SELECT concat(return_str, substr(str, old_pos, pos-old_pos), str_rep) INTO return_str;
            SELECT pos + CHAR_LENGTH(needle) INTO pos;
            SELECT pos INTO old_pos;
            SELECT locate(lower_needle, lower_str, pos) INTO pos;
        END WHILE;
        SELECT concat(return_str, substr(str, old_pos, CHAR_LENGTH(str))) INTO return_str;
        RETURN return_str;
END$$
 
DELIMITER ;
 
SELECT replace_ci( 'mySQL', 'M', 'M' );