Counts the number of occurrences of every byte-value (0..255) in $string and returns it in various ways.
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 unique characters is returned.
- 4 - a string containing all not used characters is returned.
mixed count_chars ( string $string [, int $mode ] )