从 http://www.nowamagic.net/librarys/veda/detail/2353 转载收集而来的功能函数，代码确实很好，转过来一方面是学习，另一方面做个备份：

<?php
function RemoveXSS($val) {
	// remove all non-printable characters. CR(0a) and LF(0b) and TAB(9) are allowed
	// this prevents some character re-spacing such as <javascript>
	// note that you have to handle splits with n, r, and t later since they *are* allowed in some inputs
	$val = preg_replace('/([x00-x08,x0b-x0c,x0e-x19])/', '', $val);

	// straight replacements, the user should never need these since they're normal characters
	// this prevents like <IMG SRC=@avascript:alert('XSS')>
	$search = 'abcdefghijklmnopqrstuvwxyz';
	$search .= 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
	$search .= '1234567890!@#$%^&*()';
	$search .= '~`";:?+/={}[]-_|'\';
	for ($i = 0; $i < strlen($search); $i++) {
		// ;? matches the ;, which is optional
		// 0{0,7} matches any padded zeros, which are optional and go up to 8 chars

		// @ @ search for the hex values
		$val = preg_replace('/(&#[xX]0{0,8}'.dechex(ord($search[$i])).';?)/i', $search[$i], $val); // with a ;
		// @ @ 0{0,7} matches '0' zero to seven times
		$val = preg_replace('/(&#0{0,8}'.ord($search[$i]).';?)/', $search[$i], $val); // with a ;
	}

	// now the only remaining whitespace attacks are t, n, and r
	$ra1 = Array('javascript', 'vbscript', 'expression', 'applet', 'meta', 'xml', 'blink', 'link', 'style', 'script', 'embed', 'object', 'iframe', 'frame', 'frameset', 'ilayer', 'layer', 'bgsound', 'title', 'base');
	$ra2 = Array('onabort', 'onactivate', 'onafterprint', 'onafterupdate', 'onbeforeactivate', 'onbeforecopy', 'onbeforecut', 'onbeforedeactivate', 'onbeforeeditfocus', 'onbeforepaste', 'onbeforeprint', 'onbeforeunload', 'onbeforeupdate', 'onblur', 'onbounce', 'oncellchange', 'onchange', 'onclick', 'oncontextmenu', 'oncontrolselect', 'oncopy', 'oncut', 'ondataavailable', 'ondatasetchanged', 'ondatasetcomplete', 'ondblclick', 'ondeactivate', 'ondrag', 'ondragend', 'ondragenter', 'ondragleave', 'ondragover', 'ondragstart', 'ondrop', 'onerror', 'onerrorupdate', 'onfilterchange', 'onfinish', 'onfocus', 'onfocusin', 'onfocusout', 'onhelp', 'onkeydown', 'onkeypress', 'onkeyup', 'onlayoutcomplete', 'onload', 'onlosecapture', 'onmousedown', 'onmouseenter', 'onmouseleave', 'onmousemove', 'onmouseout', 'onmouseover', 'onmouseup', 'onmousewheel', 'onmove', 'onmoveend', 'onmovestart', 'onpaste', 'onpropertychange', 'onreadystatechange', 'onreset', 'onresize', 'onresizeend', 'onresizestart', 'onrowenter', 'onrowexit', 'onrowsdelete', 'onrowsinserted', 'onscroll', 'onselect', 'onselectionchange', 'onselectstart', 'onstart', 'onstop', 'onsubmit', 'onunload');
	$ra = array_merge($ra1, $ra2);

	$found = true; // keep replacing as long as the previous round replaced something
	while ($found == true) {
		$val_before = $val;
		for ($i = 0; $i < sizeof($ra); $i++) {
			$pattern = '/';
			for ($j = 0; $j < strlen($ra[$i]); $j++) {
				if ($j > 0) {
					$pattern .= '(';
					$pattern .= '(&#[xX]0{0,8}([9ab]);)';
					$pattern .= '|';
					$pattern .= '|(&#0{0,8}([9|10|13]);)';
					$pattern .= ')*';
				}
				$pattern .= $ra[$i][$j];
			}
			$pattern .= '/i';
			$replacement = substr($ra[$i], 0, 2).'<x>'.substr($ra[$i], 2); // add in <> to nerf the tag
			$val = preg_replace($pattern, $replacement, $val); // filter out the hex tags
			if ($val_before == $val) {
				// no replacements were made, so exit the loop
				$found = false;
			}
		}
	}
	return $val;
}
?>

从代码中也可以看出，这里过滤的核心方法就是preg_replace()的正则替换，所以，过滤效果的好坏也在于正则表达式的质量高低了。