php.net/manual/en/function.iconv.php<?php
$text = "'И', 'Э'";
echo 'Original : ', $text, PHP_EOL;
echo 'TRANSLIT : ', iconv("UTF-8", "ISO-8859-1//TRANSLIT", $text), PHP_EOL;
echo 'IGNORE : ', iconv("UTF-8", "ISO-8859-1//IGNORE", $text), PHP_EOL;
echo 'Plain : ', iconv("UTF-8", "ISO-8859-1", $text), PHP_EOL;
?>
результат
Original : 'И', 'Э'
TRANSLIT : '?', '?'
IGNORE : '', ''
Plain : <br />
<b>Notice</b>: iconv(): Detected an illegal character in input string in <b>[...][...]</b> on line <b>7</b><br />