PHP: fix bug in serialization code

php/monoformat_structured.php

@@ -26,7 +26,7 @@ function serializePixels(array $pixels): string {
 function unserializePixels(int $n, string $serialized): array {
     $nBytes = intdiv($n + 7, 8);
     if (strlen($serialized) != $nBytes) {
-        $actual = strlen($serializd);
+        $actual = strlen($serialized);
         throw new \ValueError("Could not unserialize a pixel bitmap: the size $actual does not equal the expected $nBytes");
     }
     $result = array_fill(0, $n, false);