如果类存在，用php删除div

I have 2 div's, if one is shown with a class="goodbye" - how do I remove the other div with php?

Or do I have to use jQuery?

<!-- this div should not show if the below class="goodbye" is on the page -->
<div class="hello">
  Hello
</div>

<div class="goodbye">
  goodbye
</div>

Javascript, not PHP.

if ($('.goodbye').length > 1) {
    $('.hello').hide();
}

You can't do it in PHP once the page is rendered... Use jQuery instead. There are lots of different ways to do this.

Use .is(":visible")); to see if the class is visible or not.

Try this example:

$('#clickme').hover(function () {
    $('.hello').hide();
    $('.goodbye').show();
    alert($('.goodbye').is(":visible"));
}, function () {
    $('.goodbye').hide();
    $('.hello').show();
    alert($('.goodbye').is(":visible"));
});

JSFiddle Demo

PHP, being a server-side scripting language, can't manipulate the DOM. If the condition you're using to evaluate the display of your <div>'s is processed server-side, then you could use PHP to echo one <div> or the other. Otherwise, use jQuery or JavaScript to manipulate the DOM client-side.

To answer the direct question. Remove it using PHP:

if($hello)    {
    echo "<div class=\"hello\">Hello</div>";
}   else   {
    echo "<div class=\"goodbye\">goodbye</div>";
}

You can't do this with PHP since it is a server side language. Once the page is rendered you'll have to use a client side language. Yes, you can use jQuery(Javascript):

//when to handle..
$("input").click(function() {
    $(".hello").toggle();
    $(".goodbye").toggle();
});

http://jsfiddle.net/kfhb7gzq/

Here is a CSS option using the sibling selector. If .hello is sibling to .goodbye: display: none;

.goodbye + .hello {
    display: none;
}

<div class="goodbye">
    goodbye
</div>

<div class="hello">
    Hello
</div>

This solution requires reordering the elements because the sibling selector doesn't select the previous.