When converting a string from or to unicode, an invalid or unconvertable
character may be encountered. When the conversion error mode is set to
U_CONV_ERROR_SUBST, the character that caused the
error is substituted by a character set by this function. The default
substitution character is
This function is EXPERIMENTAL. The behaviour of this function, the name of this function, and anything else documented about this function may change without notice in a future release of PHP. Use this function at your own risk.