验证码一般是前端验证还是后台生成啊

我想弄个注册的验证码,但是不清楚用js做成前端生成的好还是php后台生成的好呢?请朋友们给个意见啊
谁懂呀

<html>
<head>
<script language="javascript" type="text/javascript">
var code ;
function createCode(){
code = new Array();
var codeLength = 4;
var checkCode = document.getElementById("checkCode");
checkCode.value = "";
var selectChar = new Array(2,3,4,5,6,7,8,9,'A','B','C','D','E','F','G','H','J','K','L','M','N','P','Q','R','S','T','U','V','W','X','Y','Z');
for(var i=0;i<codeLength;i++) {
var charIndex = Math.floor(Math.random()*32);
code +=selectChar[charIndex];
}
checkCode.value = code;
}
function validate () {
var inputCode = document.getElementById("yzm").value.toUpperCase();
if(inputCode != code ){ alert("验证码错误!"); return false; }
else { alert("验证码正确!"); return true; }
}
</script>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
</head>
<body onLoad="createCode();">
验证码:<input type="text" id="yzm" size="5" />
<input type="button" id="checkCode" onClick="createCode()" title="刷新验证码" style="width:50px;color:#F00;border:0;letter-spacing:1px;font-family:Arial;" />
<input type="button" id="Button1" onClick="validate();" value="确定" />
</body>
</html>
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