First, you should always use the WordPress AJAX methods, not a custom function for that. See AJAX in Plugins in the Codex.
With that practice in mind, you can set up your request like this. Change the AJAX URL to
<?php echo admin_url('admin-ajax.php'); ?>
and add the 'action': key with the value of the specific WordPress function name that you want to be executed in the next step, when the server-side receives your data.
$(".post_thumbnail").click(function () {
var id_post = $(this).attr('post_id');
$.ajax({
type: 'POST',
url: '<?php echo admin_url('admin-ajax.php'); ?>',
data: {
'post_id': id_post,
'action': 'f711_get_post_content' //this is the name of the AJAX method called in WordPress
}, success: function (result) {
alert(result);
},
error: function () {
alert("error");
}
});
});
Now we need to tell WordPress what to do, when someone calls f711_get_post_content.
Register the Action in WordPress AJAX. This is done in your Pluginfunctions. The first part (‘wp_ajax_’) is to tell WordPress that this is an AJAX action, and the part after that is the name of the action (‘f711_get_post_content’). The second argument is the function WordPress executes when this action is called.
add_action( 'wp_ajax_f711_get_post_content', 'f711_get_post_content_callback' );
// If you want not logged in users to be allowed to use this function as well, register it again with this function:
add_action( 'wp_ajax_nopriv_f711_get_post_content', 'f711_get_post_content_callback' );
After that you create your callback function. Remember to ALWAYS die() your AJAX functions. If your function outputs JSON, which I would recommend, you can quit your function by using wp_send_json( $array );, which has built in die().
function f711_get_post_content_callback() {
// retrieve post_id, and sanitize it to enhance security
$post_id = intval($_POST['post_id'] );
// Check if the input was a valid integer
if ( $post_id == 0 ) {
echo "Invalid Input";
die();
}
// get the post
$thispost = get_post( $post_id );
// check if post exists
if ( !is_object( $thispost ) ) {
echo 'There is no post with the ID ' . $post_id;
die();
}
echo $thispost->post_content; //Maybe you want to echo wpautop( $thispost->post_content );
die();
}
This would be the recommended JSON version. It enables you to pass multiple variables back to the client.
function f711_get_post_content_callback() {
// retrieve post_id, and sanitize it to enhance security
$post_id = intval($_POST['post_id'] );
// Check if the input was a valid integer
if ( $post_id == 0 ) {
$response['error'] = 'true';
$response['result'] = 'Invalid Input';
} else {
// get the post
$thispost = get_post( $post_id );
// check if post exists
if ( !is_object( $thispost ) ) {
$response['error'] = 'true';
$response['result'] = 'There is no post with the ID ' . $post_id;
} else {
$response['error'] = 'false';
$response['result'] = wpautop( $thispost->post_content );
}
}
wp_send_json( $response );
}