Jquery $ .ajax（）POST

I cannot seem to get data from an array using the $.ajax() function. I am displaying a failure message when access the data name.

$.ajax({
    method: "POST",
    url: "update.php",
    data: { content: $("#textbox").val()}
})
.done(function( msg ) {
    alert( "Data Saved: " + msg );
});

The above code works but when the code in update.php is not pulling the msg and triggering Failure

<?php
if(array_key_exists('content', $_POST)){
    echo "Success";
} else {
    echo "Failure";
}
?>

the name: 'content' should display the content in #textbox but it does not.