Ajax调用中的PHP变量

I have to create a site that looks like tumblr and instagram and shows all pictures which are stored in one directory. When you scroll to the end of the page I want to load new pictures but I am a Ajax beginner and dont have a clue how to do this. I think my program will work but I need a PHP variable in the Ajax call. How can I do this?

You can simply echo it into the script the same way you would echo it onto an html page

<script>
...
var phpvar = <?= $php_variable ?>;
...
</script>