mb_substitute_character() specifies substitution character when input character encoding is invalid or character code is 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_detect_encoding() and mb_send_mail().
substchar : Specify Unicode value as integer or specify as string as follows
"none" : no output
"long" : Output character code value (Example: U+3000,JIS+7E7E)
Return Value: If substchar is set, it returns TRUE for success, otherwise returns FALSE. If substchar is not set, it returns Unicode value or "none"/"long".