The API does what it's supposed to do, it would be smart to add error checking.

API.php

@@ -11,35 +11,46 @@ function longLatZip($zip){
     $param = Array(0 => $zip);
 
     $result = resultQuery($sql, $param);
-    $retVal = False;
+    $retVal = False;//delete this line
     if($result->num_rows > 0){
         $retVal = $result->fetch_assoc();
         $result->close();
+        //return here
+    }else{
+        //throw an error here
     }
     return $retVal;
 }
 
 /*
-calculating 
+Finds all the zip codes within maxDistance miles of zip.
 https://stackoverflow.com/questions/24370975/find-distance-between-two-points-using-latitude-and-longitude-in-mysql/24372831#24372831
 */
 function nearbyZips($zip, $maxDistance){
     $zipArr = longLatZip($zip);
     if(!$zipArr) return false;
-    $sql = "SELECT a.city AS from_city, b.city AS to_city, 
-    69.0 *
-     DEGREES(ACOS(COS(RADIANS(a.Latitude))
-          * COS(RADIANS(b.Latitude))
-          * COS(RADIANS(a.Longitude - b.Longitude))
-          + SIN(RADIANS(a.Latitude))
-          * SIN(RADIANS(b.Latitude)))) AS distance_in_mi
-   FROM city AS a
-   JOIN city AS b ON a.id <> b.id
-  WHERE a.city = 3 AND b.city = 7";
-  $params = [];
-
+    $sql = "SELECT P AS zipCode, 
+        ROUND(
+            69.0 *
+            DEGREES(ACOS(COS(RADIANS(LAT))
+            * COS(RADIANS(?))
+            * COS(RADIANS(LNG - ?))
+            + SIN(RADIANS(LAT))
+            * SIN(RADIANS(?))))
+            ,2) AS distance_in_mi
+        FROM zip
+        HAVING distance_in_mi <= ?";
+    $params = [$zipArr["LAT"], $zipArr["LNG"], $zipArr["LAT"], $maxDistance];
+    $result = resultQuery($sql, $params);
+    $retVal = [];
+    while($row = $result->fetch_assoc()) {
+        $retVal[]=$row;
+    }
+    $result->close();
+    return $retVal;
 }
 
-print_r(longLatZip(20194));
-?>
-HELLO WORLD!
\ No newline at end of file
+if(isset($_GET["zip"]) && isset($_GET["dist"])){
+    echo json_encode(nearbyZips($_GET['zip'], $_GET["dist"]));
+}
+?>
\ No newline at end of file