大家写插件嘛

/*
* 引入jQuery库，然后引入这个插件，调用如下：
 jQuery('input[placeholder]').placeholder();

jQuery.fn.placeholder = function(){
    var i = document.createElement('input'),
        placeholdersupport = 'placeholder' in i;
    if(!placeholdersupport){
        var inputs = jQuery(this);
        inputs.each(function(){
            var input = jQuery(this),
                text = input.attr('placeholder'),
                pdl = 0,
                height = input.outerHeight(),
                width = input.outerWidth(),
                placeholder = jQuery('<span class="phTips">'+text+'</span>');
            try{
                pdl = input.css('padding-left').match(/\d*/i)[0] * 1;
            }catch(e){
                pdl = 5;
            }
            placeholder.css({'margin-left': -(width-pdl),'height':height,'line-height':height+"px",
            'position':'absolute', 'color': "#cecfc9", 'font-size' : "14px",'display':'inline'});
            placeholder.click(function(){
                input.focus();
            });
            if(input.val() != ""){
                placeholder.hide();
            }else{
                placeholder.css("display","inline");
            }
            placeholder.insertAfter(input);
            input.keyup(function(e){
                if(jQuery(this).val() != ""){
                    placeholder.hide();
                }else{
                    placeholder.css("display","inline");
                }
            });
        });
    }
    return this;
};