Ajax success if statement and echo mySQL query result - php

Question

I have an Ajax script that makes a call to a php file on my server every twenty seconds.
The server then runs a simple mysql query to return the contents of a particular field.
If that field is blank I want the php file to echo the word "pending", which when caught by the success handler will recall the initial function. However if that field is not blank, it will contain a URL to which I want to redirect the user to. That field will update any where between 5 seconds and 5 minutes from the start of the first call and that time cannot be changed.
I think the main issue may be with my php file, in that I dont think it is echoing the data in a way that the success handler recognises. However I have detailed both parts of my code as whilst the success handler seems to be constructed correctly I am not 100% sure.
Very new to this, so apologies if I have not explained myself correctly but if anyone could assist that would be great:
UPDATE - for clarity what I am looking to achieve is as follows:
Ajax call to my php file.
PHP file queries database
If field queried contains no data echo the word "pending" to the ajax success handler (IF) which in turn recalls the original function / ajax call.
If field queried contains data (will be a URL) echo this result to the ajax success handler (ELSE)in a format that will redirect the user via window.location.assign(data).
FURTHER UPDATE
I managed to solve this question with using a combination of the advice from #mamdouhalramadan and #martijn
I also have changed setInterval to setTimeout as the poll function was causing responses to stack up should the server be running slowly and as such cause errors. I also added in cache: false and a further option in the success handler to take into account slightly different behaviour in IE:
AJAX
function poll() {
$.ajax({
url: 'processthree.php?lead_id='+lead_id,
type: "GET",
cache: false,
async: false,
success: function(data3) {
//alert("pending called " + data3)
if(data3.indexOf("pending") >-1 ){
setTimeout(poll, 20000);
}
else if ( (navigator.userAgent.indexOf('MSIE') != -1) ) {
//alert("Submit success - MSIE: " + data3);
parent.window.location.replace(data3);
}
else{
//alert("process three called " + data3)
window.top.location.assign(data3);
}
},
error: function(xhr, error){
//alert("Error");
//alert("Error: " + error + ", XHR status: " + xhr.status);
},
});
}
setTimeout(poll, 20000);
PHP
$query = ("SELECT column FROM table WHERE id = '$lead_id'") or die(mysql_error());
$result = mysql_query($query);
$return = array();
while($row = mysql_fetch_assoc($result))
{
$return = 'pending';
if($row['column'] != '')
{
$return = $row['column'];
}
}
echo $return;

Answer 1

I believe using json might help you out here, not to mention it is safer, like so:
function poll() {
$.ajax({
url: 'processthree.php?lead_id='+lead_id,
type: "GET",
dataType: 'json',//specify data type
success: function(data3) {
if(data3.res.indexOf("pending") >-1 ){
//rest of the code.....
then in your php:
$return = array();
while($row = mysql_fetch_assoc($result))
{
$return['res'] = 'pending';
if($row['column'] != '')
{
$return['res'] = $row['column'];
}
}
echo json_encode($return);
Note: use PDO or MYSQLI instead of mysql as it is deprecated.