php if statement doesn't work with jquery variable

The jQuery variable I send to my PHP doesn't work (Or atleast, it doesn't seem to work ). I've sent it to my php with ajax.

Please take a look at it, perhaps you can see the problem:

                $('.do').click(function(){
                var cid2 = $(this).attr('id');
                var gebridauthpos = cid2.indexOf('||');
                var gebridauth = cid2.substring(gebridauthpos+2);
                $.post("agenda.php", {gebridauth: gebridauth});
                alert(gebridauth);
                <?php
                    if ($admin == true || isset($_POST['gebridauth']) AND $_SESSION['id'] == $_POST['gebridauth']) {
                        echo "$('#dialog').dialog('open');\n";
                        echo "var cid = $(this).attr('id');\n";
                        echo "var d开发者_JAVA技巧atum = cid.substr(0, 10);\n";
                        echo "var naampos = cid.indexOf('|');\n";
                        echo "var gebridpos = cid.indexOf('||');\n";
                        echo "var naam = cid.substring(naampos+1,gebridpos);\n";
                        echo "var gebrid = cid.substring(gebridpos+2);\n";
                        echo "$.ajax({\n";
                            echo "type: \"POST\",\n";
                            echo "url: \"agenda.php\",\n";
                            echo "data: naam,\n";
                            echo "success: function(){\n";
                                echo "$('#gebruikerinput').html(\"<input type='text' READONLY='' size='35' value='\" + naam +\"'>\");\n";
                                echo "$('#gebridinput').html(\"<input type='hidden' name='gebridtextbox' value='\" + gebrid + \"'>\");\n";
                                echo "$('#datuminput').html(\"<input type='text' READONLY='' size='12' name='datum' value='\" + datum + \"'>\");\n";
                            echo "}\n";
                        echo "})\n";
                        echo "return false;\n";
                    }
                ?>
            });

Basically what I want to do, is using "gebridauth" in the if statement of my PHP when I click on a TD. If the TD is the same as the person that's logged in, show the dialog.

You need a callback on your $.post call, right now you're just sending off the POST and not paying any attention to the what the server sends back so no dialog will appear. I think you want something more like this (with real code where the big comment is):

$.post("agenda.php", {gebridauth: gebridauth}, function(data, textStatus, jqXHR) {
    // If the server sent back a "show the dialog" value in data then
    // show the dialog and all the other stuff that's currently in a
    // bunch of PHP echo calls.
});

I think you're misunderstanding how AJAX works. You can't mix Javascript and PHP like that, since they're running at completely different times on different systems. If you're POSTing to agenda.php, your PHP code needs to be in the file agenda.php. That file should not contain Javascript. You also won't be able to echo Javascript in return like that.