幻灯片中使用的ajax get方法

I'm making a slideshow with the get ajax method, but I'm a little confused on how it works. When I click next shouldn't in run through the gallery_.php file with index incremented which would change the slide?

PHP/HTML

if (!isset($_GET["index"])) {
    $index = 0;
}
else{
    $index = $_GET["index"];
}
echo "<div id='slider'>
        <ul class='slides'>

            <li class='slide'>
                <div class='pic'>
                    <img src= " . $dir . $pic_array[$index] . " />
                </div>
                <div class='caption'>
                    <p id='title'>$titles[$index]</p>
                    <p id='des'>$descriptions[$index]</p>
                </div>
                <div class='next'>
                    <i class='fa fa-arrow-right fa-2x'></i>
                </div>    
                <div class='previous'>
                    <i class='fa fa-arrow-left fa-2x'></i>
                </div>
           </li>";

echo     "</ul>  
      </div>


      </html>";

Javascript

$(function () {
    var arrPix = $('#json_pics').val();
    var arrPix = $.parseJSON( arrPix );


    var index = 0;

    var $slider = $('#slider');
    var $slides = $slider.find('.slides');
    var $slide = $slides.find('.slide');
    var $next = $slides.find('.next');
    var $previous = $slides.find('.previous');
    var $caption = $slides.find('.caption');

    var slide_length = $slide.length;

    $slider.hover(function() {
            $caption.css('opacity', '1');
            $next.css('opacity', '1');
            $previous.css('opacity', '1');
        }, function() {
            $caption.css('opacity', '0');
            $next.css('opacity', '0');
            $previous.css('opacity', '0');
        }
    );
    $next.click(function() {
        $.get("gallery_.php?index=" + 1);
        index++;
    });
});

Right now you're doing

$.get("gallery_.php?index=" + 1); 

and concatenating a number to a string ends up as the string

$.get("gallery_.php?index=1"); 

this is what you're sending every time, you wanted to do

$.get("gallery_.php?index=" + index);