Ajax创建XMLHttpRequest对象通用方法

function createXmlHttp()
{
	var xmlHttp;
	try{
		xmlHttp = new XMLHttpRequest();		// Firefox,Opera 8.0+,Safari
	}catch (e){
		try{
			xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");		// Internet Explorer 6.0+
		}catch (e){
			try{
				xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");		//Internet Explorer 5.5+
			}catch (e){
				alert("您的浏览器不支持AJAX！");
				return false;
			}
		}
	}
	return xmlHttp;
}