If the optional split_length parameter is specified, the returned array will be broken down into chunks with each being split_length in length, otherwise each chunk will be one character in length.
If split_length is positive, splitting will be started from the begining of the string, otherwise splitting will be started from the ending of the string.
If the split_length length exceeds the length of string, the entire string is returned as the first (and only) array element.
If the split_length length equal to 0 or has incorrect format, split_length will be setted to 1 (default value);
FALSE is returned if str is not a string.
> str_split('This is test string');
[ "T", "h", "i", "s", " ", "i", "s", " ", "t", "e", "s", "t", " ", "s", "t", "r", "i", "n", "g" ]
> str_split('This is test string', 6);
[ "This i", "s test", " strin", "g" ]
> str_split('5963986', -3);
[ "5", "963", "986" ]