Eliminar caracteres no UTF8 de una cadena

utf8.php

// Supposing your want to be sure that $string is composed by valid UTF-8 characters:
$string = iconv("UTF-8","UTF-8//IGNORE",$string);

// If you want to remove none ISO-8859-1 characters from this string:
$string = iconv("UTF-8","ISO-8859-1//IGNORE",$string);
$string = iconv("ISO-8859-1","UTF-8",$string);

// If you still have characters you want to be ignored, try this:
$string = recode_string("us..flat", $string);