(PHP 4 >= 4.0.6, PHP 5)
mb_substitute_character — Set/Get substitution character
   Specifies a substitution character when input character encoding is invalid
   or character code does not exist in output character
   encoding. Invalid characters may be substituted NULL (no output),
   string or integer value (Unicode character code value).
  
This setting affects mb_convert_encoding(), mb_convert_variables(), mb_output_handler(), and mb_send_mail().
   If substchar is set, it returns TRUE for success,
   otherwise returns FALSE. 
   If substchar is not set, it returns the current
   setting.
  
Example #1 mb_substitute_character() example
<?php
/* Set with Unicode U+3013 (GETA MARK) */
mb_substitute_character(0x3013);
/* Set hex format */
mb_substitute_character("long");
/* Display current setting */
echo mb_substitute_character();
?>