... PHP 4 >= 4.0.0) count_chars -- Return information about characters used in a string . mixed count_chars ( string string [, int mode]) . ... 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. ...