PHP站点如何防御XSS攻击呢？看下面的过滤XSS攻击的PHP函数吧，很实用，下面的函数可以用来过滤用户的输入，保证输入是XSS安全的。具体如何过滤，可以参看函数内部，也有注释。

1. <?php
2. function RemoveXSS($val) {
3. // remove all non-printable characters. CR(0a) and LF(0b) and TAB(9) are allowed
4. // this prevents some character re-spacing such as <java\0script>
5. // note that you have to handle splits with \n, \r, and \t later since they *are* allowed in some inputs
6. $val = preg_replace('/([\x00-\x08,\x0b-\x0c,\x0e-\x19])/', '', $val);
7. // straight replacements, the user should never need these since they're normal characters
8. // this prevents like <IMG SRC=@avascript:alert('XSS')>
9. $search = 'abcdefghijklmnopqrstuvwxyz';
10. $search .= 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
11. $search .= '1234567890!@#$%^&*()';
12. $search .= '~`";:?+/={}[]-_|\'\\';
13. for ($i = 0; $i < strlen($search); $i++) {
14. // ;? matches the ;, which is optional
15. // 0{0,7} matches any padded zeros, which are optional and go up to 8 chars
16. // @ @ search for the hex values
17. $val = preg_replace('/(&#[xX]0{0,8}'.dechex(ord($search[$i])).';?)/i', $search[$i], $val); // with a ;
18. // @ @ 0{0,7} matches '0' zero to seven times
19. $val = preg_replace('/(&#0{0,8}'.ord($search[$i]).';?)/', $search[$i], $val); // with a ;
20. }
21. // now the only remaining whitespace attacks are \t, \n, and \r
22. $ra1 = Array('javascript', 'vbscript', 'expression', 'applet', 'meta', 'xml', 'blink', 'link', 'style', 'script', 'embed', 'object', 'iframe', 'frame', 'frameset', 'ilayer', 'layer', 'bgsound', 'title', 'base');
23. $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');
24. $ra = array_merge($ra1, $ra2);
25. $found = true; // keep replacing as long as the previous round replaced something
26. while ($found == true) {
27. $val_before = $val;
28. for ($i = 0; $i < sizeof($ra); $i++) {
29. $pattern = '/';
30. for ($j = 0; $j < strlen($ra[$i]); $j++) {
31. if ($j > 0) {
32. $pattern .= '(';
33. $pattern .= '(&#[xX]0{0,8}([9ab]);)';
34. $pattern .= '|';
35. $pattern .= '|(&#0{0,8}([9|10|13]);)';
36. $pattern .= ')*';
37. }
38. $pattern .= $ra[$i][$j];
39. } //phpfensi.com
40. $pattern .= '/i';
41. $replacement = substr($ra[$i], 0, 2).'<x>'.substr($ra[$i], 2); // add in <> to nerf the tag
42. $val = preg_replace($pattern, $replacement, $val); // filter out the hex tags
43. if ($val_before == $val) {
44. // no replacements were made, so exit the loop
45. $found = false;
46. }
47. }
48. }
49. return $val;
50. }
51. ?>