... non è completo ma ci và vicino...
... non uso il tag [ p h p ] altrimenti gli slash vanno via
... per copiarti il codice fai il quote del mio commento e copialo da lì...
<?php
$char_special = array(
"\"" => "&quot;",
"&" => "&amp;",
"\'" => "&apos;",
"<" => "&lt;",
">" => "&gt;",
"\u00A0" => "",
"\u00A1" => "&iexcl;",
"\u00A2" => "&cent;",
"\u00A3" => "&pound;",
"\u00A4" => "&curren;",
"\u00A5" => "&yen;",
"\u00A6" => "&brvbar;",
"\u00A7" => "&sect;",
"\u00A8" => "&uml;",
"\u00A9" => "&copy;",
"\u00AA" => "&ordf;",
"\u00AB" => "&laquo;",
"\u00AC" => "&not;",
"\u00AD" => "­",
"\u00AE" => "&reg;",
"\u00B0" => "&deg;",
"\u00B1" => "&plusmn;",
"\u00B2" => "&sup2;",
"\u00B3" => "&sup3;",
"\u00B4" => "&acute;",
"\u00B5" => "&micro;",
"\u00B6" => "&para;",
"\u00B7" => "&middot;",
"\u00B8" => "&cedil;",
"\u00B9" => "&sup1;",
"\u00BA" => "&ordm;",
"\u00BB" => "&raquo;",
"\u00BC" => "&frac14;",
"\u00BD" => "&frac12;",
"\u00BE" => "&frac34;",
"\u00BF" => "&iquest;",
"\u00C0" => "&Agrave;",
"\u00C1" => "&Aacute;",
"\u00C2" => "&Acirc;",
"\u00C3" => "&Atilde;",
"\u00C4" => "&Auml;",
"\u00C5" => "&Aring;",
"\u00C6" => "&AElig;",
"\u00C7" => "&Ccedil;",
"\u00C8" => "&Egrave;",
"\u00C9" => "&Eacute;",
"\u00CA" => "&Ecirc;",
"\u00CB" => "&Euml;",
"\u00CC" => "&Igrave;",
"\u00CD" => "&Iacute;",
"\u00CE" => "&Icirc;",
"\u00CF" => "&Iuml;",
"\u00D0" => "&ETH;",
"\u00D1" => "&Ntilde;",
"\u00D2" => "&Ograve;",
"\u00D3" => "&Oacute;",
"\u00D4" => "&Ocirc;",
"\u00D5" => "&Otilde;",
"\u00D6" => "&Ouml;",
"\u00D7" => "&times;",
"\u00D8" => "&Oslash;",
"\u00D9" => "&Ugrave;",
"\u00DA" => "&Uacute;",
"\u00DB" => "&Ucirc;",
"\u00DC" => "&Uuml;",
"\u00DD" => "&Yacute;",
"\u00DE" => "&THORN;",
"\u00DF" => "&szlig;",
"\u00E0" => "&agrave;",
"\u00E1" => "&aacute;",
"\u00E2" => "&acirc;",
"\u00E3" => "&atilde;",
"\u00E4" => "&auml;",
"\u00E5" => "&aring;",
"\u00E6" => "&aelig;",
"\u00E7" => "&ccedil;",
"\u00E8" => "&egrave;",
"\u00E9" => "&eacute;",
"\u00EA" => "&ecirc;",
"\u00EB" => "&euml;",
"\u00EC" => "&igrave;",
"\u00ED" => "&iacute;",
"\u00EE" => "&icirc;",
"\u00EF" => "&iuml;",
"\u00F0" => "&eth;",
"\u00F1" => "&ntilde;",
"\u00F2" => "&ograve;",
"\u00F3" => "&oacute;",
"\u00F4" => "&ocirc;",
"\u00F5" => "&otilde;",
"\u00F6" => "&ouml;",
"\u00F7" => "&divide;",
"\u00F8" => "&oslash;",
"\u00F9" => "&ugrave;",
"\u00FA" => "&uacute;",
"\u00FB" => "&ucirc;",
"\u00FC" => "&uuml;",
"\u00FD" => "&yacute;",
"\u00FE" => "&thorn;",
"\u00FF" => "&yuml;",
"\u0152" => "&OElig;",
"\u0153" => "&oelig;",
"\u0160" => "&Scaron;",
"\u0161" => "&scaron;",
"\u0178" => "&Yuml;",
"\u0192" => "&fnof;",
"\u02C6" => "&circ;",
"\u02DC" => "&tilde;",
"\u03A9" => "&Omega;",
"\u03C0" => "&pi;",
"\u2013" => "&ndash;",
"\u2014" => "&mdash;",
"\u2018" => "&lsquo;",
"\u2019" => "&rsquo;",
"\u201A" => "&sbquo;",
"\u201C" => "&ldquo;",
"\u201D" => "&rdquo;",
"\u2020" => "&dagger;",
"\u2021" => "&Dagger;",
"\u2022" => "&bull;",
"\u2026" => "&hellip;",
"\u2030" => "&permil;",
"\u2039" => "&lsaquo;",
"\u203A" => "&rsaquo;",
"\u2044" => "&frasl;",
"\u20AC" => "&euro;",
"\u2122" => "&trade;",
"\u2202" => "&part;",
"\u220F" => "&prod;",
"\u2211" => "&sum;",
"\u221A" => "&radic;",
"\u221E" => "&infin;",
"\u222B" => "&int;",
"\u2248" => "&asymp;",
"\u2260" => "&ne;",
"\u2264" => "&le;",
"\u2265" => "&ge;",
"\u25CA" => "&loz;");

$string = "Grazie mille per l'aiuto, \u00ec \u003 \u00fc \u00e8";
echo str_ireplace(array_keys($char_special), $char_special, $string);

?>