Improve checkCompatibilityWithExistingValue comment

Author: chuckdries

lib/utils.ts

@@ -216,8 +216,13 @@ function checkCompatibilityWithExistingValue(
             isCompatible: true,
         };
     }
-    // An empty array existing value is compatible with an object update.
-    // PHP's json_encode produces [] for empty associative arrays that should be {}.
+
+    // PHP's associative arrays cannot distinguish between an empty list and an
+    // empty object, so it encodes both as []. A key that should hold an
+    // object may arrive from the server as [] and be stored that way. If
+    // we then try to MERGE an object into that key, the array-vs-object type check
+    // would normally block it. Since an empty array carries no data worth
+    // preserving, we treat it as compatible with an object update and coerce it.
     const isObjectValue = typeof value === 'object' && !Array.isArray(value);
     if (Array.isArray(existingValue) && existingValue.length === 0 && isObjectValue) {
         return {isCompatible: true, isEmptyArrayCoercion: true};