Syntax of count_chars in PHP
mixed count_chars(string string[, int mode])
Returns the number of occurrences of each byte value from 0–255 in string; mode determines the form of the result. The possible values of mode are:
0 (default) Returns an associative array with each byte value as a key and the frequency of that byte value as the value
1 Same as above, except that only byte values with a nonzero frequency are listed
2 Same as above, except that only byte values with a frequency of zero are listed
3 Returns a string containing all byte values with a nonzero frequency
4 Returns a string containing all byte values with a frequency of zero