Counts the number of occurrences of every byte-value (0..255) in string and returns it in various ways. The optional parameter Mode default to 0. Depending on mode count_chars() returns one of the following:
0 - an array with the byte-value as key and the frequency of every byte as value.
1 - same as 0 but only byte-values with a frequency greater than zero are listed.
2 - same as 0 but only byte-values with a frequency equal to zero are listed.
3 - a string containing all used byte-values is returned.
4 - a string containing all not used byte-values is returned.