fix to return correct transparent png

tileserver.php

@@ -433,7 +433,7 @@ public function getCleanTile($scale = 1, $format = 'png') {
         header('Access-Control-Allow-Origin: *');
         header('Content-type: image/png');
         // 256x256 transparent optimised png tile
-        echo unpack('H', '89504e470d0a1a0a0000000d494844520000010000000100010300000066bc3a2500000003504c5445000000a77a3dda0000000174524e530040e6d8660000001f494441541819edc1010d000000c220fba77e0e37600000000000000000e70221000001f5a2bd040000000049454e44ae426082');
+        echo hex2bin('89504e470d0a1a0a0000000d494844520000010000000100010300000066bc3a2500000003504c5445000000a77a3dda0000000174524e530040e6d8660000001f494441541819edc1010d000000c220fba77e0e37600000000000000000e70221000001f5a2bd040000000049454e44ae426082');
         break;
     }
     die;